The motor coupling that connects the motor to the gears has broken. In the early models this coupling was made of a very hard clear plastic that would become very brittle over the years to pass. Later models were fitted with a black plastic coupling that doesn't deteriorate over time.
At any given time this brittle plastic can crack and when the coupling breaks it leaves the mirror in the upright position, like it would be when making an exposure. The motor will keep on running as there is no signal that the cycle of the camera has finished.
For this to be fixed the camera will have to be dismantled and the coupling will be replaced.