You should be able to advance to where you where by tapping on the screen once (after the video starts up again) and tap on the multiple square icon on the right. Once in the menu, you can scroll down to the place you were in your workout before your device locked up. Let firstname.lastname@example.org know if you have any other issues with your device - they're happy to help!
I have an android tablet. If it locks up, I have to restart the entire workout, go into the menu then I can select the start of the workout section but I can't get back to the specific move. There's no "fast forward" feature.