It depends a lot on how it was defeated, what its origin was and how badly defeated it was.

If the defeat was bad, the victor will conduct both a battlefield pursuit, killing or capturing as many as possible of the enemy soldiers - a majority of deaths usually happened once one side had started running during this era. If the army did not dissolve after the battle and at least some of its leadership was intact, it might attempt to retreat to a nearby stronghold, or try to unite with reinforcements or allied forces it knew was on their way beforehand. To prevent this, a victor would conduct a post-battle pursuit, attempting to destroy the enemy army before it could reform and regain moral, cohesion and combat ability.

A defeated army that dissolved under the pressure of pursuit could splinter into various groups - some which might attempt to retreat or find friendly forces to join up with, others that may take up banditry and pillaging the countryside. If the army that was defeated was in hostile lands (and guilty of looting, burning, murdering, raping, pillaging, plundering - none of which was exclusive to enemy lands, plenty of armies did that in their own land as well), the fleeing soldiers might very well face a very angry populace out for revenge and be hunted like animals.

Mercenaries that were captured were often offered service to the victorious side in exchange for their life and eventual freedom, and towards the end of the medieval era it was not uncommon that the victorious army emerged larger from the battle due to absorbing prisoners.

Lords and knights captured would often be held for ransom, while commoners might be executed, offered service or sometimes released. Holding a large amount of prisoners was often hard, as they required supplies and guards that the victorious army was unable or unwilling to provide.