Do you mean the power light is still on or you can't turn on your computer? If the former is the case, check the case wire connections on the motherboard. You may have them in the wrong spot. If the latter is the case, something is shorting out your computer and most likely, it's lint from the paper towel you used. You need to use a lint-free cloth for that instead. Still, check the screws (make sure they don't come in contact with the top of the motherboard) or see if any of the thermal grease may have fallen onto the motherboard upon putting your heat sink and fan back on.