Hello,
Most of the water heaters have 2 reset buttons. One is the electrical safety which is generally located outside of the box underneath. Just press it, it's absolutely safe ( avoid being barefoot into a wet shower)
The second one is the overheating safety. This one is again generally located above the boiler. It's a very tiny almost invisible little button. In this case, wear gloves, isolate yourself from ground and just press on it...job done your boiler should be operational again.