Seems to be the dreaded problem of 2007-2008 MacBook pro motherboard Graphics Card,
To prove this Switch on your laptop turn upside down and place between a heavy towel or duvey cover from bed for about 30minutes take out switch off hold down on button then let cool down before uprighting and turning on again if your laptop works it means it has the graphics card problem you need to level the damper under the graphics card on motherboard as the heat has allowed the pins to lose contact with the motherboard or get a professional to repair for you apple has been known to charge from £200 and above so get someone else and price some online people will do for about £90 try ebay you need to post motherboard to them to repair and they give you a fixed price over phone before repairing for you.
Jerbel123