Bafang does not assist

My Prime with 1000km is perfect. I took the ebike for a km review and I had to change the chain and sprockets due to wear. When I went out to test it, everything was going correctly until the engine stopped assisting. The display marked the assistance mode but the motor did not work. However, the walk mode does provide assistance... Turn it off and when it turned on it worked perfectly. I turned it off again and stopped attending. Any advice? thank you!