Although I do not have too much experience yet (5 or 6 batches so far) ... I would guess that, because the airlock blew off, all of the built up pressure went away... It will prob take some time to build back up enough pressure to start bubbling
But keep in mind (I've read this a lot of places) just because it isn't bubbling doesn't mean it isn't fermenting..
If you took a hydrometer reading when you put it in the primary check it after the required time the recipe requested for fermenting and see if it has dropped accordingly
+1 to the previous post. Airlock activity (or lack there of) can be very misleading. An airlock is simply that, a barrier for air and carbon dioxide. Its not a tool for identifying fermentation. Sometimes you can have a complete fermentation and never see airlock activity. Hydrometer is the only way to go.