All I did was blow out 3 years worth of dust with an air compresser (and plugged in another memory stick which has been removed) and now I can't even get it to power up.  I'm a hardware guy, but without any schematics etc, I'm lost.  I've checked all the connections and jumper's and nothing.  Even substituted another power supply with no success.  I need a starting point.  Thanks, CT in VirginiaMore info please on the type of COMPUTER, motherboard ,proccessor ,age any peripherals drives etc..
check that all the connectors, ide cables etc. are firmly pushed into drives and motherboard. check that any fans fitted are working and the powersupply unit is switched on    ( its a SIMPLE thing but I have found it to be switched of and forgotten to be switched back on on many occasions)
the main danger with an air duster is what you point it at and how you point it, you can easily blow of jumper connecters and then disable the pc.
Try removing all LEADS from the back of the pc,
Hold the power button in for around 20 seconds,
reconnect the cables and try booting up again.

If that fails, remove the CMOS battery for about 20 seconds, refit and try again.In ADDITION to above, as a starting point:

- remove the newly added memory until you have your system working again (as it was before your cleanup).

Dell