I recently noticed a loud tapping sound coming from my exterior a/c unit. The a/c was working great but the noise was very loud so I opened the top to remove a piece of plastic that was being hit by the fan. After replacing the top the a/c blows, but is not cold. I didn't do anything but open it and close it and now its not working. The plastic didn't appear to be a major component, just something that lined the top of the unit. Any suggestions?

You may have knocked a thermostat wire loose, open up the unit and check wire connections.

I recently noticed a loud tapping sound coming from my exterior a/c unit. The a/c was working great but the noise was very loud so I opened the top to remove a piece of plastic that was being hit by the fan. After replacing the top the a/c blows, but is not cold. I didn't do anything but open it and close it and now its not working. The plastic didn't appear to be a major component, just something that lined the top of the unit. Any suggestions?
Usually there is a "micro switch" in the channel of the plastic cover that has to be depressed to allow the air conditioner to run. If you don't have the cover properly seated or it is otherwise not depressing the switch the machine will not run no matter what you do.