This sounds a bit mad, but to check whether it is a read/write issue due to heat. Try turning it on and then even if it doesn't boot, leave it on for it to warm up.

The 'platters' of a hard drive (disks where the data is stored) do expand or contract depending on their temperature, so if you have shut it down while it was roasting and then gone for a boot up when it is cold, it may not find the sectors it is after.

I remember 'back in the days' (floppy disks and LS120 disks are the same) I would put a uni assignment on a disk (that I had probably been up all night, writing last minute! ) and then walk the 2-3 mile journey to class, in the standard Scottish weather (snow!). On inserting the now half frozen disk into the machine it would not read, however half an hour later once it was warm, it was fine.

Worth a try!

If by some miracle this works, download the free program 'speedfan' and see what temperature your system and HDD is running at. This may highlight a poorly performing fan or poor air circulation.