My electricity went off during the night and I woke up as it came back on. The light on the furnace flashes four times which says it means 'limit switch open'. I need help.

Have you tried to shut the power off to the furnace for a few minutes and the back on? It might have been in the middle of a heat cycle when the power went off causing the problem. Some limit switchs are manual reset. Without model or make of furnace it is hard to help you.

Eric D

The main limit should have reset itself after the unit cooled down, what is probably still tripped / open is the roll-out safety switches, which are next to the burners. To rest them, just push in on the center button, on both of them, then repower up the furnace, and it should run.