A readied action can be prevented and/or disrupted

A readied action takes a specific trigger. "I attack if he attacks me", or "I ready an action to attack when Joe the Bard casts a spell". In any case, if that trigger is not met until your next turn, you won't do anything with your readied action. So, if your bard was incapacitated or otherwise made to not be able to cast a spell (or if he decided not to), you wouldn't attack. Once your turn comes back around, you can choose to act normally, or ready your action again in hopes that it triggers this time. The "old" readied action is discarded.

A readied action *can* be prevented from happening even if the trigger is met. This is because readied actions happen immediately *after* the trigger condition is met. Therefore, if you were reduced to 0 hit points by a trigger, were incapacitated/paralyzed/charmed, or otherwise made to be unable to attack (or whatever your readied action was), your readied action would be interrupted/prevented. 

Example: You declare a readied action to attack an Orc if he attacks you, because you are not sure if he is hostile. The orc attacks you and reduces you to 0 hit points. At this point, your trigger condition is met, but you are unable to make an attack because you are unconscious.  

*Counterspell* can also interrupt any readied spell.