I woke up this morning and it was cold so I went to turn on our oil/radiator heater plugged in the Family room and it wouldn't turn on. I noticed that the T.V. and Sat. box were off too, also the aquarium on the next wall. They worked perfectly fine the night before. I used my handy outlet tester and it says the Hot/Ground is reversed. How can it just do that out of the blue? And How can I fix the problem?

This is an indication of an open neutral condition on the circuit. This can happen for several reasons, but heavy draw loads, like a space heater, can tax iffy connections. Connections like back-stabs in the back of receptacles are such connections.

Remember, the problem could be in any receptacle or splice on the circuit. It is not necessarily on the one the heater is plugged into.