Humidity sometime can cause this - put some silica gel packs inside of your cabby or if you have Ac in the room turn it off, open the doors and after an hour or so start the comp.
Check HDD and ODD and the cables and ports on the mobo - if you have any USB device attached to the comp remove it it and try starting the comp.
BTW, tell us what GFX card, mobo, cpu and PSu you are using.
Mention manufacturer and model names of those and mobo bios version.