If your answer was given  in the bounty period and it has earned 2 upvotes but it was not accepted, then after the bounty period has ended + 24 hours, you will receive half of the bounty.

This automatic mechanism is useful for where there is a good answer as recognised by the community but the bounty offerer has not themselves  manually offered the bounty:

 - Let's say the bounty offerer did not come back in the 7 days - the community mechanism takes care of this issue.

 - Let's say the bounty offerer did not think the answer given was the answer they were  looking for - again this community mechanism recognizes the effort put into the answer

The bounty offerer and the original poster does not have to be the same - the bounty  offerer has the "control" on this.  Thus, if the OP never revisits their question, its up-to the bounty offerer to either manually give you the bounty, or yourself to give a great answer that persuades everyone else to upvote you.

If you did not receive more than 2 upvotes, then the bounty will not be awarded.

More info about the bounty system can be found:

http://meta.stackoverflow.com/questions/16065/how-does-the-bounty-system-work