yamaha g100 1x12 version 3, amp issue - no sound (sometimes)

[phatt]

You have not mentioned that you soldered the failed joint?
SO,,,
If the solder joint was loose as you noted and you have NOT re-flowed that joint, then it only takes a bump and the amp will fail again.
To fix the issue you will need to remove the whole PCB to access the solder side and re-flow the failed solder pad. If you don't feel confident in doing the above then you will need to take it to a repair shop as it's a tricky job.
Phil.