This should be fixed now - there was (as one of the error messages purported) a crashed MySql table.
I've repaired the table.
It's the biggest table on the forum so when it goes wrong it can affect many things. When a user makes a new post, any new words are logged so that the search can find them at a later date.
It's probably about time the whole search index was rebuilt anyway, I've found over time it goes a bit funny (that's a technical term).