If your hot water is not working when temperatures outside have been low, you could have a frozen pipe. Signs of a frozen pipe are:
There is no water from the hot taps or only a trickle
You can visibly see a pipe is frozen over
You notice odd smells coming from taps or drains
If you find a pipe frozen, you can try thawing them by pouring warm (never boiling water as it can cause thermal shock) over them or wrap a hot water bottle around them until they defrost.