I had this problem suddenly arise on just one site in a HostGator reseller account. I tried the suggested theme-switching thing, but it didn’t work.
I contacted HG and suggested that perhaps the php.ini needed updating, as @pafoster suggested, but the tech replied with:
“This is not due to php.ini. This is due to hitting Rlimits on the server. I have raised the rlimits to allow the user to use the full 256 MB that we allow.”
This seems to have solved my problem.
Also want to reiterate what @gnarzilla wrote. I’ve had a Hostgator reseller account for several years now, and on the rare occasion that I’ve had problems, they’ve provided excellent customer service and have solved my problems very quickly. In fact, I’d say that I can’t think of a single online business that has service as good.
H.