List Computer specifications. (Specific Power supply model needed.)
Is this a prebuilt? If so just List Company and Model.
If its not a prebuilt or the warranty is already voided you can Open the case, (While computer is off) Unplug everything except the 4 Pin and 20 PIn power connector to motherboard, Front panels connectors, RAM Sticks, Video card and CPU.
This is a POST test, once the RAM, CPU and video card are the only things left in, you can turn it on and see if it POSTs. If it doesn't your looking at Dead motherboard, RAM, CPU or Video card.
Report back before further explaination.