By the Information you have given, it is likely an ATX system.
If you opened and assembled a PC earlier, you can do the following:
Otherwise ask someone who did.
ALLWAYS UNPLUG THE SYSTEM WHEN TAKING OUT AND REINSTALLING HARDWARE!!
FAILURE TO DO SO WILL RESULT IN IRRECOVERABLE DAMAGE!
Unplug the power to the PC.
Open the PC.
Ground yourself and regualairly tuch the metal casing of your PC to keep you free of Static buildup. (It is FATAL to computer components if you became Statically charged while handling PC parts!)
Press ALL the expantionboards One at a time to verify that they are firmly seated in their slots.
(Also the Memory chips!)
Press only SRAIGHT TOWARDS THE MAINBOARD!
Failure to do so may result in damage to your expansion slot and/or the card!
If you have a Slot-1 or Slot-A CPU, Remove it, Clean the SECC contacts with an antistatic dry cloth, blow out the ventilator and the Cooler-body, and replace the CPU.
If it is a Socket CPU, leave it alone.
You may only blow out the ventilator and the Cooler-body if youn seem to be fit.
Plug in the power, and try if it will bootup.
If OK, close the system and you are done.
Of NOT OK, unplug the power, and strip your system to its bare minimum.
Only the CPU, Memory (ONE chip), and your Videocard should stay in place.
Tip: To avoid Windows to redetect all the hardware, Replace the cards in their EXACT slot where they came out when reinstalling them!.
If done, Plug in the power and reboot.
If OK, place ONE of your epansion board in.
(Memory first if you had more chips installed!)
Continue this with any left boards.
If failure reoccours after installation of a board, you have just reinstalled the Failed board.
Remove that board and replace it with a new one.
If NOT OK on its bare minimum, exchange your memory chip with another (If you had more chips installed)
If not and after replacing of ALL the components still not, bring it to a computer service point to have it checked out.