[Paul M]

Have you ever installed the vB3 MAPI (for mobile) plugin ?

That had a bug at one point that broke the style table in such a way that it would cause the above;

If you run the SQL below, it should fix it.
Before you do, back-up your styles, as you may lose one of them (the last one you created).

DELETE FROM style WHERE styleid = 0;
ALTER TABLE style CHANGE styleid styleid SMALLINT NOT NULL AUTO_INCREMENT;

(Note, if you use a table prefix, you will need to change the table name above to match).