Easiest to check would be the battery. Either have it checked, charge it, or try jumping the car. When your checking the battery make sure the terminals are tight and are not corroded. If that isn't the problem it sounds like a bad ground or loose wire (including battery terminal). If it's a bad ground they can be a bit of a pain in the ass to track down, again your looking for corrosion or a loose wire. I know you already mentioned the starter but, have you taken it out and had it tested? Hope that helps a bit.