Our water cooler won't dispense water when the water jug is on it

Unplug it, first, to see if it's a frozen line. If so, you may have to adjust the thermostat or replace it. There are also water valves and a filter screen that can stop up. Look for a line filter that has a hex head cap that goes in at an angle. Remove it and the screen comes out. Of course, turn off water first. There can be more than one screen in some water fountains. Give us the model number and brand so we can look it up.