As Dave said check the engine earth first, if it does turn out to be that check the throttle is free afterwards, as the cable can melt, because the engine will use the throttle cable as an alternative earth, as its so small it gets hot and melts causing the throttle to stick.
If it's not that take the battery to a garage and get them to check the battery with a battery tester as it my read over 12.4 volts on a multi meter but still have a dead cell causing you a problem.
Once I had a car that wouldn't turn over, everything checked out fine until I took the plugs out, turns out the cylinders had filled up with antifreeze as the head gasket was leaking into the engine.
