I can think of a number of reasons, none of which would be fixable by the average user:
Magnet knocked off centre trapping the voice-coil
Burned voice-coil due to overloading
failed connection between speaker terminal and voice coil
1 or more failed components in the crossover unit
Some hi-fi speakers have an overload protection fuse which could have failed ( this is the only one I can think of that you should be able to fix yourself).
You assume that the problem is with the speaker; to be certain you should swap the left and right speakers and check that problem follows the speaker - if not then clearly the problem is with the amp.