Today, when I tried to log on to ExHentai.org, it kept showing the sad panda. It was working fine just the other day.

Check if you're logged in on e-hentai.
Check if all the cookies for exhentai are deleted.
u3u

One of your cookies expired. It always happens eventually. You just need to do what Psi said.

expired cookies, as mindwipe explained, an extra advice from me is to use a cookie editor addson on your browser to change the expiration date to 2020 :)

How do I check if all the cookies are deleted for that website alone?

Never mind I managed to get it working again somehow.