Took cover off my tower

Lets tackle the monitor first: A no signal message indicates one of two things;

• The video card isn't properly seated; (if the video card is on board skip to next step) just moving the video cable on the monitor to unplug it is enough to unseat the video card. Open the tower again and make sure the card is firmly in place.
• If the video is onboard, the plug may not be firmly in place. Check the screws that hold the plug in place and see if they are turned as far as they will go; there's a chance the plug is only partially making contact giving you the no signal message

As far as the mouse goes, the only reason the machine won't recognize the mouse would be a faulty connection in the USB port. Assuming no damage was done while blowing the dust out, double check the USB port, maybe even changing ports to see if the mouse comes back. If your machine is anything like mine, space behind the tower can get kinda cramped and just putting it back into place and reconnecting the power cord can pull just enough on the lines to pull a line just enough to disrupt your connection.

Hope this answers your questions. Best of luck.
Greg