A common reason for this is that the lock has not fully engaged possibly due to the door having "dropped".
To confirm this try lifting the handle fully with the door in an open position and see if the cylinder will turn - if it does there is a problem with the amount of travel in the doorlock when closed i.e. not aligning woth the lock keep.
If it doesn,t the the problem is with either the lock or the cylinder
Replacements for which can be found at www.lockfinder.co.uk
Sparky
October 2009