I haven't looked under the rewind crank assembly on an F5, but since it uses a lot of the parts from the F4, it's probably the same. There's a spring that goes between the lock lever and a pin. It probably has popped off. To fix it, you need to remove the crank and the plate that's underneath the crank.
Removal/installation (for an F4 - like I said, probably the same):
1) Open back and remove it from the body. 2) Flip crank out, exposing screw. 3) While using one screwdriver inserted in the teeth of the rewind shaft, unscrew the screw holding the crank on. 4) Use snap ring pliers to expand the snap ring. Remove snap ring 5) Remove plate 6) Locate loose spring, hook it onto the lock lever, then, use a small slotted screwdriver to hook the other end onto the pin that's to the left. Lever pivots on the pin that's to the right. 7) Reinstall plate, snap ring, and crank. 8) Test function of the lock lever. If all is good, then proceed to #9 9) Reinstall back.
Hope this helps. Removing the back is important, since, to gain access to the back lock, if the rewind shaft drops into the body, requires removing the rewind side top cover. Had that happen on my previous F4s, when I was reverting it back to its original crank, so I could keep the nice one for my "new" F4, which needed one.
D1x, F4s, F2AS, F2A, F FTn, FM2n, N8008, Nikomat FT2, FTn - John Laughlin Photography - nature photographer