It isn’t possible to find out for certain why Harold is missing. All anyone can do is offer some suggestions of what might be stopping him from visiting. Once the hogs have left our gardens their lives inevitably become a bit of a mystery.
Some possible reasons are, if Harold happened to be a she, she could be off somewhere having hoglets, or someone on the route to you may have been doing some DIY on their fences and blocked holes the hog was using. Also someone else nearby may have started feeding, so that he/she may not need to visit your garden. Males have larger ranges than females and Harold might be spending a bit of time in another part of the range. All sorts of possibilities.
Hopefully Harold will eventually return. It would be nice if there were some hoglets in tow, but then you might have to have a name change!