Toiler wrote:
I accidentally ran a li-ion battery completely flat and now it won't
charge. Is there any way to revive it?

Regards,
toiler

Nope, even if you could get is to take charge, the capacity would minimal.
Li Ion need strict managagement both in terms of voltage and current
during charge and discharge. Play by it's rules and they are very good.
The better packs have management built in and give very good service.