Make sure that you have set the switch on the bottom to "On". Otherwise, the massage gun will not turn on.
Make sure that the battery of the massage gun is charged. Before the first use, you should fully charge the battery. Always use the original power supply to do this.
When you use the OPOVE M3 Pro for a long time at a stretch, it is possible that it has overheated. In this case, leave the massage gun switched off for at least an hour before using it again.
If this won't help and you still have a warranty on the OPOVE M3 Pro, contact the manufacturer with your problem. They can help you in most cases.