Sorry to hear the hog has disappeared. I know it’s very distressing when a favourite hog disappears. I keep telling myself not to get too attached to individual wild hedgehogs, but sometimes it just can’t be helped.
It’s possible that the fox killed it, but not necessarily the case. Some foxes seem to leave hogs alone, but others will kill them and, being predators, none of them can really be trusted around hogs.
It may be that the hog got a fright and decided it was time to move somewhere else. They do tend to move nests from time to time, anyway, not least to avoid a build up of ticks, fleas, etc. But it also may be more warily approaching, i.e. staying under the cover of bushes, etc.
You say the fox was back the next night, but has it been back since? I’m assuming you have cameras to be sure the hog isn’t visiting? Although cameras cannot be totally relied on to prove a hog was not there.
In case the hog or another one comes back, you might like to make a fox proof feeding area, so that any hogs which might turn up can eat in peace. They are likely to be more vulnerable when eating. If it’s placed near to cover they are more likely to be able to escape. Also a fox is less likely to visit if it can’t get access to any food.