There are a few possibilities for this.
My friend had the same problem and it turned out his motherboard died. Couldn't figure out why until he told me that he used a feather duster to dust his computer
Power supply could be dead, it happens. Get a paper clip, unplug the 20/24 pin mobo connector and stick the paper clip in with the green wire and any black wire. It should
power on. If it doesn't, an obvious sign it's gone, or a wire is shorting out (recently happened to my dad's PC).
Have you checked the power button and the connection itself? Make sure it's plugged in (I don't think polarity matters, check anyway to be safe) and make sure no wires got disconnected somewhere.
Hope that helps.