Alright, I think you mean that you turned the fan off at a switch. And now it won't turn back on (?)
First check any fuses or circuit breakers (your electrical power supply to your building), that may have tripped (interrupted the power to the fan switch).
If that doesn't fix the problem, make sure the power is off TO the fan assembly, and open the box attached to
the fan housing. There is probably a large fuse inside that box that may have blown).
Look at the electrical connections in that fan box to see if you see any heat damage that may have occurred that
might explain the failure. The electrical connections, maybe it's own switch, and the electrical connections that go
into the fan.
If you have building power to the switch (the 'socket'), and you have power through any fuse in the fan housing electrical box, you may have an electrically 'open' thermal protection device as a part of the fan itself. The only way
to determine that is to find somebody with a voltmeter, who can trace the electrical power through everything I just
talked about, and see where the actual power interruption to the fan is at.
Hope you find this problem easily, but it's an electrical device, and you may need a voltmeter to find the problem.