I am sure you have checked this but:
1) check the bypass valve to be closed. If it is open, cold water can mix with the hot and you will not be happy.
2) check the furnace. If the furnace does not fire up, it could be low gas pressure.
3) press the reset buttons on the hot water heater. Two rubber coated buttons in the upper left corner.
Let me know if that fixed it.
Lou and Laura with Bella - German Short Hair Pointer
2008 GMC Sierra 2500HD Crewcab SB Allison Duramax
2010 Flagstaff 8526RLWS - Superglide 3300
HAM CALLSIGN - KC3FFW