Your EPG is probably updated by overnight downloads. (My Freeview box works that way). If the signal strength wasn't strong enough when your TV attempted to accept the download, you'll simply see an empty EPG. (I write from experience. The wind blew my aerial into its current, rather precarious, position. Ever since then the EPG has nearly always been blank, because the signal strength hasn't been good enough for the download - even though I can normally receive most channels with no problems).
Take a look at your aerial to see if it's been knocked out of position. If not, the failed download was probably due to a temporary problem (such as high atmospheric pressure or a solar flare). Your EPG will probably reappear tomorrow (after receiving the latest download). However you might be able to reduce the chances of the problem reoccurring by using a cheap signal booster.
Chris