If your hot water has stopped running, there are a few things you can do to try and fix the issue. First, check to see if the water heater is turned on. If it is, then check to see if there is power running to it. If the water heater is turned off or not receiving power, then it will need to be serviced by a professional.
If the water heater is turned on and receiving power, then the next step is to check the pilot light. If the pilot light has gone out, then you will need to relight it. Once the pilot light is lit, you should then be able to turn on your hot water again.
If you have checked all of these things and your hot water still isn't running, then it's likely that there is a problem with your plumbing. You'll need to call a plumber to come and take a look at your system in order to determine what the problem is and how best to fix it.