Assuming it has a lot of miles on it...
My guess is that the lack of oil, itself, didn't cause a major problem. There's likely been enough wear over time (and increased clearances) that it can tolerate being very low on oil.
It won't be happy, of course.
The question is, why was it low on oil? Since you don't see evidence of a leak, my guess is that your intake valve seals are leaking and you're burning the oil. Do you see a puff of blue smoke when you start the engine in the morning?
If it won't turn over after this, and after it has a chance to cool down, well then it could be pretty serious. Meaning new crank bearings and things like that. I would think that the pistons and rings would probably be Ok (due to the increased clearance over time).
Fingers crossed that it's something simple and cheap.
Cheers,
Scott.
