the obligatory...Are you logged in?
This happens to me all the time also but I think it's because I've started wearing my mother's old bi-focals -- and my neck is so sore from going up and down.
It's been an ongoing problem for at least a year. It doesn't happen very often, and every thread that acted up eventually did load, even if you had to wait and retry after an hour or so.
(It happens either with Mozilla or IE, so it's something on the site giving the problem).
Ha! It was Cyber Sitter! I turned it off and it stopped screwing things up.
A few times when I've hit search I've gotten a forbidden HTML result. And yes, i am logged in.
Generally just trying it again a few times does the trick. Never seen this happen before today though.