Is the PC powering up at all? Fans running? Does the keyboard show the numlock & caplock lights? Is the mouse lighting up? If so, sounds like a video issue. Check to be sure you didn’t accidentally disconnect any cables, etc. when you were cleaning the fans. Also, if the PC seems to be powering up & has other video outputs (VGA, for example) and you have a VGA monitor, or VGA input on your usual monitor, try connecting that way. If you have a separate video card, could be you bumped it out of the slot slightly. Make sure all cards are fully seated into their slots.