In short **Yes**.

There is lot of examples for retirement players who made a "comeback".

When a player decides to retire it his personal decision, He can do what ever he decides to do.  
In case the player decides to do a "comeback" we can divide the "comeback" to 2 categories:

 1. Personal sport (Like Tennis) - The player can register to the any tournament like a other  "unretire" player, e.g. [Kim Clijsters][1] who announced retirement from professional tennis on [2007][2] and made a comeback on [2009][3] and even return to to the 1st place in the WTA rankings.

 2. Team sport (Like basketball) - The player can announce on "comeback" and need to find a team that will want him, or that his international team will called him up to play like every other Free Player. The best example is [Michael Jordan][4] who retired 3 times! 2 from basketball and 1 from Baseball.

You can read about other athletes who made a comeback at the following [discussion][5].

[1]: http://en.wikipedia.org/wiki/Kim_Clijsters
[2]: http://en.wikipedia.org/wiki/Kim_Clijsters#2007:_Retirement
[3]: http://en.wikipedia.org/wiki/Kim_Clijsters#2009:_Comeback
[4]: http://en.wikipedia.org/wiki/Michael_Jordan
[5]: http://forum.santabanta.com/showthread.htm?277955-15-Athletes-Who-Retired-and-Came-Back