I accidently hit my dell Optiplex 755 computer and the screen froze and made a buzzing sound. I either turned the computer off or unplugged the computer cable and plugged it back in. The pc went on and so did the keyboard, however the mouse didn't go on and the monitor went on for a few seconds, but then a message appeared and it said that it cannot find a signal or the monitor cable. The cable was already plugged in properly. I unplugged everything and plugged everything back in and still the mouse and monitor won't work and the pc also beeps for a few seconds, when turning on.

Edit: The part where I plug in my HDMI cable and my monitor cable is pushed further back into the PC. Could that be a problem? 

What do I do?