What's stopped working? The whole system? Task Manager just showing 4 cores? You need to give us more information.
Why don't you download and run Prime95 and see how many workers it fires off - if you get 8 then Hyperthreading is enabled as you have 1 worker per thread. If you only get 4 then Hyperthreading is disabled or not recognised and you're getting 1 worker per core.