It is entirely possible he had a stroke, seizure, or there is something else going on (PT or hind leg degeneration, even simply an arthritis flare-up). How old is he? Does he have any other symptoms? Has he shown anything else unusual, even if minor, over the last little while? You say dehydration is a consideration; has he been avoiding water?
If you're concerned, a vet visit is always recommended; and often they can advise what happened and what can be done to help ease or manage any current symptoms.