The search function disables automatically when the server load threshold is reached. Searching is very CPU and disk intensive (probably because the posts on this site are not fully indexed [that is why sites like GOOGLE, AltaVista, etc., seem to seach the Internet so fast - they search their indexes]). Things get worse when when lots of searches are concurrent.
Like everyone above posted, wait until the demand is less (early in the AM or late in the PM).
The good news is these boards are very popular, and there could be literally 100's of DIS'ers online at any time, and 1000's of Dis'ers / visitors stopping by here per day. That a wealth of information for your reasearch!