Some users are experiencing an unfortunate bug that crashes the game after playing Jam mode, and the game won't restart. This is a known bug and fixing it is our top priority. We'll be rolling out an update very soon.
In the meantime, deleting and reinstalling the game will make Mad Skills BMX 2 playable again. When you do this, simply reconnect to the same social media account you originally connected to and all of your progress and purchases will be restored.