Upon changing a light fixture for my gf, light was only controlled by breaker, switches had no effect. There are three pairs of wires (one black, one white) entering the light fixture box. I removed all nuts b/c they were cracked and very old, and replaced with new ones (in hindsight...stupid). The wiring is very old with this stringy stuff wrapped around the core. Probable asbestos aside, on the fully functional old (but not pretty) light, light could be turned on/off at either switch. When new light was installed, I nutted off two sets and connected a third to the light. Realizing that switches weren't working, I nutted off the the pair I had been using (all three pairs now nutted) and tried to close the breaker and test for which pair was hot, but now the breaker keeps tripping. How can I fix this? Note, haven't touched any switches, and wiring is probably consistent with 50s/60s techniques.
One of the pairs, the one that you originally connected to the light, are the hot and neutral from the breaker box. When you connected them together, you created a short circuit. This is why the breaker won't reset.
Probably one of the others is the switch loop, connected to the light switch on the other end. You need to identify this. If you have a ohmmeter or continuity tester, do the following: WITH THE POWER OFF, connect the meter to the wire pairs until you find the one that opens and closes with the switch. This is the switch loop.
Connect one wire of the switch loop to the hot wire from the breaker panel. Connect the other wire to the hot side of the light. Connect the neutral from the breaker box to the neutral side of the light.
The question I cannot answer is what the other pair are used for. If you fix the light but leave the third pair disconnected (put a wire nut on each wire separately), are there any other outlets or light that don't work? If yes, you probably need to connect these wires also to the hot and neutral from the breaker box. This is used to carry the electricity to the other loads.
If everything is working, just leave these wires disconnected.
Tip: next time you disconnect wires, mark them all with tape to note how they were connected. This may save you a bill for an electrician's visit.