Stuck/failed check valve on HOT water outlet (nipple on tank....check valve is inside)
You can disconnect HOT water line at tank.then remove nipple and remove internals (just break them up with a screw driver/pliers) then re-install nipple and HOT water line.
You will then have hot water......pick up a new check valve when convenient OR better yet a shutoff valve and do away with that junk check valve.
Check valve is there to stop water from backflowing into WH tank via HOT line WHEN WH is bypassed (winterizing etc>)