From your description of the hard drive light staying on constantly and nothing happening, that doesn't sound like a normal boot up sequence to me but with no video - sounds like a motherboard issue (would've thought it's unlikely to be the power supply.)
Before you fear the worst though, I'd check everything over for potential shorts - it wouldn't be the first time I've seen a broken cable or a bare cable touching another part causing this sort of an issue (especially as the original problem was a loose cable.) As suggested earlier, I'd also unplug everything but the bare essentials to check if a dodgy component is playing havoc with everything else.
Either way I don't really see the point in power supply testers these days - if you're suspicious just swap it with a new one and see if it solves the issue
Failing all that though, my guess would be the motherboard.