Here I am.
If it's electric, check the breaker box and flip it back and forth like reba said because they can trip without flipping the switch, but no power comes through them/
If it's gas, take the lower cover plate off and look in it to see if the pilot is burning or not. If it is, dial the temperature control up to a hotter setting to see if you can make the burner come on. If the pilot isn't burning, light it. Turn the top knob to Pilot position, push it down and hold it down and reach in and light the pilot. Continue holding down the gas knob for 60 seconds or so and then release it, it should stay on, then turn the knob back around to the ON position which should bring on the burner.
If you're not comfortable lighting the pilot, most utilities companies will come light it for free, or maybe grab a neighbor to help.
That'll be 89.95 please....
post back if you have any questions. There are some newer water heaters out there with electronic ignition, wiring and so forth. I'd have to know what you have to be able to help with it, but I could! I'd just need the model and serial number and the brand.