That happened to me once when my bow dry fired because of a broken nock, and it surprised me to the point that I let the bow fly out of my hand, and the riser cracked when it hit the pavement. The bowyer cut me a good deal on a new riser, which has worked fine ever since. To me, that's the best solution possible. No bowyer wants to think that one of his risers would crack on its own, and I’m sure he appreciated me telling him the true story. Only an expert could tell you if it might be possible to repair the one you have.