As Ratchet Freak says, it's going to blow away or cause the engine to fail. Latter might happen if it sits tight enough that it doesn't get blown away before overpressure develops in the engine.
Not a good thing, damage may happen.
My guess most likely outcome is that it will be blown back, possibly colliding with parts of the aircraft (and thus causing damage) or hitting the hangar wall or whatever else is behind the aircraft with some speed (unless there's a lot of clear area, in which case it'd flutter to the ground), if it's not overfastened.
And to be complete, it's a cover to prevent things from getting into the engine while the aircraft is parked. Dust, animals, anything.
There will be others on the air intakes as well.