No worries. It’s difficult when the same problem shows up, and from Googling this topic I can see that it’s widespread.
I just flushed cache/deleted cookies in both Chrome and Explorer and tried the sites, and THEY ALL WORK. (The original problem was also showing up in other browsers, by the way.)
I can only put it down to reinstalling the files over the existing ones (by the way, I didn’t delete the old folders, either… just replaced them: all but wp-content) and then flushing the cache, since nothing else I did had any effect. (Unless Hostgator has somehow fixed the problem on a server-wide basis, of course.)
Whatever the reason it’s working, I’m breathing a big sigh of relief.
Marg