The "pop" may have been the cable pulling loose in its bracket clamp. Put the der in its rest position and see if there is slack in the cable. If there is excessive cable slack while in the rest position (usually on the small chain ring) the cable has slipped. Loosen the clamp screw, grab the cable end with a pair of pliers, gentle pull it to remove the slack and tighten the clamp bolt while keeping the cable taught. It's easier than it sounds. Keep us in the loop and good luck to you.
For learning how to fix "old school" bikes, pick up a copy of "Any body's bike book" by Tom Cuthbertson. I still ride an '84 old school Peugeot (bought it new) and Tom's book has kept it going all these years.
Jon