Has ther EVER been hot water? soind to mr like a plumbing problem, so find the closest hot water spigot to the tank, and if that is warm there may be misrouted plumbing. or if this has just happened it might be a control board or thermal sensor that is signaling the burner/heating element to shut of.
One thing to try carefully is heat the water to shut off and then turn off the HW heater, release the pressure in the lines (pump off and no water pressure), and then with gloves carefully unscrew the anode, and see if THAT water is hot. If it is it most certainly is the bypass or something else in the water lines, if it is only warm, it is the water heater. Be carefull and don't get scalded. make sure all pressure is off, and ALL the power and gas is shut off. AND then be real careful... and even more careful.
2007 Chevy 2500HD Classic LBZ Duramax/Allison (Stock)
2005 Bigfoot 1500C8.2 AWESOME