Air conditioner was started with cover on outside compresser. Unit was on for two hours. Not working now

May have a high pressure switch that tripped and needs reset, or if has a contol board or defrost board then the power may need rest at the indoor unit. If nether of these then you may have a more serious problem like no freon, or loss of a compressor. Does the fan outside even come on?

Could be very bad news or as simple as resetting a high pressure cut out, if the a/c was equipped with one.