A heart stopping moment! Can you reach the area beneath the 'crack' through the soundhole? Better still, can you get a view of the underside with a mirror and flashlight? If so, try to locate the exact area inside the guitar and press gently on the top from above and you should be able to detect if there is movement in the top along the length of the crack.
There is a good chance it is only a lacquer crack which is easily repaired but even if it's not, the fix is relatively straightforward.