Do you get any water flow thru faucet hot side?
No Flow......check valve in water heater hot out (top line) has failed.
Yes flow but is warm then goes cold....bypass valve on water heater is open (cold water flowing to hot water line)
Yes flow but it is cold and bypass valve is closed.......electric/propane not turning on heating water.
Do you have a RED light ON----part of On/Off switch?
RED Light is 'Fault' Indication
Could be thermalfuse blown
Could be bad t-stats
Could be blown DC fuse
Could be bad circuit board
Without more/better info just guessing.
Brand/Model of water heater?
Water flows/doesn't flow?
Details...........