As mentioned, there really isn't enough info here.
The simple answer is, just use the other hammer.
The long answer has questions in it.
Does the hammer move back to half cock, but won't stay there when released? If so, but full cock works properly, then the half cock notch in the hammer is most likely broken out.
Does the hammer stop, immediately after you start moving it back, and the cylinder doesn't turn? If so, and the cam looks good, something is amiss with the bolt. Due to tolerance differences, one cam might be slightly lower than the other, and the bolt leg is not slipping over on top of the cam of the problem hammer. If it worked before, and just started this, then the bolt leg on the hammer side has probably gotten bent down some, and you should replace and fit a new bolt.
A third option is the trigger nose has broken, but since the other hammer works properly, this isn't real likely.