Ok last weekend or so, I unplugged water heater to make a repair. After I fixed the busted line I plugged it in again and it worked fine. This weekend I unplugged it in case lines froze again wouldn't leak hot water. Tempature outside is back up so I plugged heater in and now I can't get bot water. I've checked the breaker on the wall and hit the reset button on hot water heater. What are some possible causes.

You have a plug-in water heater? What is the make and model?

I unplugged it in case lines froze again wouldn't leak hot water

You should know that unplugging or turning off a water heater will not prevent a leak if a pipe freezes and burst.

If you are concerned about a hot water pipe freezing you should close the cold water input valve to the water heater and drain the lines. Do this by closing the cold water input valve, opening all hot water faucets. Then open the drain valve of the water heater and let about 2 gallons of water out.
OR
Open all hot faucets to a fast dribble. This helps prevent pipes from freezing because the water in the pipe, at the point of freezing, is constantly being replaced by warmer water from the water heater. You can do the same for cold water pipes.