As an ever growing individual, you will see people enter your life, affect your life, and leave your life countless times. Losing someone who's helped shape your inner self can sometimes be best for both you and for them. Everyone walks their own path, and those paths just don't always go in the same direction. Other times, the situation and the circumstances surrounding it force people to part ways when maybe they were meant to walk hand in hand through eternity. If you're looking to find someone from your past there are more than a few resources at your disposal. Unless this someone has taken measures to seclude themselves, you should be able to find them.
Perhaps the easiest method of rediscovering someone with whom you used to have some sort of relationship is social networking sites such as Myspace and Facebook. The increasingly popular Twitter might be worth a look too. On any of these sites all you have to do is create a profile (simple and free) and go to the “search for friends” section (on Facebook and Myspace, hover over the friends tab at the top of the screen and click the “find friends” button that will appear below; on Twitter there is a search box in the upper right hand corner with the same purpose) and type in the first and last name of whoever you're looking for. Classmates.com (a relatively popular site which specializes in reuniting former high school classmates) is also a good resource, especially if you went to school with the person you're in search of. Social networking has had an explosion of popularity the last several years (with no noticeable setbacks to speak of) so even if you feel that the internet might not be the forte of the person who you're searching for, you should probably check.
Think about anyone you still have contact with who may still have contact with the person you're looking for. You might be surprised at how many people hang on to every single phone number they gather throughout their lives, especially now in the cell phone era. Go through your phone's contact list and check your entire Myspace and Facebook friend lists for people who knew, or may have known, that certain someone you're looking for. Sometimes finding a person from your past can be as simple as just going through someone from your present.
You can also use government records to find someone from your past. Government records are essentially comprised of all the documented events that occur in one's life. This includes birth certificates, criminal and civil litigation history, home addresses, marriage and divorce records and many other government recorded happenings. Some of these (like home addresses and birth certificates) are generally kept private but a large portion of government records are available to the public, as a way to keep accountability within democracy in check. The easiest way to gather basic public records is to do a Google search of a person's first and last name. You might uncover a person's profile on any number of social networking sites, a person's blog or website (it's not uncommon for people to use keywords and such to make their blogs more Google friendly so as to get more traffic to their site), their place of business (with a phone number), or even a home address in some cases. It takes all of 10 seconds to make the search so it's at least worth a shot. Google is far and away the most popular search engine on the web, and the wealth of information (on everything and everyone) to be found there is virtually boundless. Some sites that are a little more specialized in government record tracking (as opposed to the “everything” tracking of Google) include VitalRec (a site which gives good explanations on how to obtain some of the more confidential records not easily accessible), Zabasearch, FirstGov, PublicData (a small fee applies to this one) and DirectGov (UK specific).
If everything has failed, there's practically a guarantee that you'll find someone from your past if you hire a private investigator. If someone has died or disappeared (if you suspect, after exhausting all options short of enlisting the help of an investigator, that this could be a possibility but you'd rather not know then you should consider doing yourself a favor and letting it go), gone to jail (statistics show that people who get in trouble with the law tend to repeat the behavior) or left the country without leaving many connections behind then this may be your only hope. Private investigators have resources that the general public just can't get its hands on, and missing person cases are one of their specialties. I'm sure you can already imagine the catch here: they're extremely expensive. You can expect to be looking at at least $30.00 an hour for a private investigator's services (and up to $100.00 or more, depending on the experience and expertise of the individual investigator, and the lengths which the investigator feels that he or she will have to through to assist you). However, there are websites that can do less comprehensive (but still more comprehensive than what you can do on your own) searches for missing persons for a much smaller fee. Such sites include Legal Papers (based in Maryland but serving surrounding areas as well), Lostpeople and USSEARCH.
More often than not, finding someone from your past is really not all that difficult. Nearly everyone these days has a piece of the internet to call their own in some way or another (with Myspace and Facebook being king) and it's quite rare that anybody actually takes measures to prevent themselves from being sought out. One reason for this is that many people have a kind of hidden desire or fantasy to one day be discovered by someone from their past, so you may just be making a dream come true.