Hey Josei,
Feel the hot water pipe coming out of the heater. Is it warm all the way into the house? You must first find out if your pipes are froze.
To do this open the closest hot water faucet to the heater and thaw out the supply line from the heater to the faucet. If the line was froze you should be getting waterout of the faucet now. They make a heating tape that wrap around pipes to keep them from freezing. Let me give you a link for instructions on installing heat tape.
http://www.mygreathome.com/fix-it_guide/heat_tape.htm
Your local hardware store should carry heat tape but if they don't click on back and I'll direct you to a site that sells them. Good luck, tom