Make sure nothing obstructs the cover and that the latch is not stuck in the lock position. This video shows the removal and installation would simply be a reversal of the procedure. It should simply slide forward but again make sure nothing obstructs the cover position while sliding.
Hello, I know it may sound weird, but I'm having the hardest time trying to close the case of a Dell Optiplex 5050 MT. I just received it today (Ebay). I was able to remove the lid to check the internals. However, for some reason, I can't get it to close. It will not slide shut. I'm wondering if I'm doing something wrong. 
Close Case Dell Optiplex 5050 MT