The problem sounds like either the handle spindle which is easy to replace or the locking system which is more complicated. Check another window in the house and identify the lock, a name will be stamped on the face plate. Check the backset and how many roller cams or locking points it has. Measure the length of the lock ie the length of the face plate. It will be in metric. Search the internet to find a replacement and as suggested take the glass out, manipulate the locking points until open. Take out the handle and the locking system. Replace the lock and handle.
Paul, March 2011
