put cover valve gasket back on now it wont start why
You did the work, likely you messed something up.
That is a wild guess considering you told us nothing about the car at all except the year make and model.
You likely left a wire or hose disconnected.
An engine will usually start even if the valve cover is completely removed. I don’t advise to try this experiment btw, just saying is all. That job is very easy on my Corolla, but on the Sunfire it is considerably more complicated. Besides disconnecting the battery, involves the air outlet resonator, accel cable, pcv, fuel line brackets, brake booster, ignition coil/module, ground strap. My guess is some problem with the reconnection of the ground strap or the ignition coil/module.