I have a propane bradford white power vent hot water heater installed in 2006. It will work normally and heat water for about 12 hours then will stop lighting but the exhaust fan will continue to run. I will then unplug the unit (say overnight) then plug it back in and it will fire up and work normally until it shuts down after a period and not light back up .when it shuts down it won't fire again unless I unplug it , leave it unpluged then plug it back in and then it fires up for a period. Please help thank you

There is a long sequence of events that occur to heat your water in a power vent system. The thermostat calls for heat, starts the fan, when the fan is running the igniter heats up or sparks, the gas valve releases the gas, a flame sensor lets the igniter know that its job is done and continues to sense the flame allowing the gas valve to remain open. When the water is to temperature the thermostat shuts everything down. Somewhere in this process is most likely where your problem lies. You probably need to replace the gas valve where most of the communication center lies. It also is home to the thermostat.
I would suggest you call a plumber familiar with Bradford White heaters or if you are very handy and know your way around a multimeter, call Bradford White with your model number and serial number and they should be able to walk you through a series of diagnostic tests.
You will want to know your model number and serial number anyway because it sounds like any parts you need should be under warranty.
Good luck!