@Peter-Cheung
I think you are asking what to do if you are on the road with the BMW Connected app and need to restart the route for some reason or how to restart the route if you have deviated from the route along the way.
The app has two ways to start the route:
1: Via starting point
If you use this option, the app will create a route from your current location to the starting point of your route. You can then drive the entire planned route.
2: Straight route
If you take this choice, the route will be created from your current location to the nearest point on the route. You can then continue on the remaining planned route.
I myself often use choice 2 if, for example, if I have stopped the route to insert a gasstop or lunch/coffee stop.
(there is a third choice to start a route but I find it useless and I have not used it enough to go into it. In that case the route will be shown on your screen and you will drive there on your own).

when the BMW ride app freezes
I haven't experienced this yet.