Check on all your high ranking keywords on search engine and find out whether they are still there or have lost position.
What sort of percentage are we talking about here? Are you normally in 2, 3, 4, 5 digit pageviews? Being only half the traffic means one thing when you normally get 40 pageviews per day than it does if you normally get 4000 pageviews. Traffic fluctuates constantly, how many standard deviations below "normal" did it go down? Have you made any changes to the site? You say "from yesterday" Since it's still not even 8am google time (stats reset on Google days) I'm not sure how you can make any qualified statement with less than a third of the day gone by.