It depends on why it stopped. If it fails because of an auth error at PIA, then OpenVPN considers that fatal and exits. We have a fix for that on 2.4.1 and later (using "auth-retry nointeract")
If there is something else causing it to exit, then the fix would be different. Have to see the error in the OpenVPN logs to know for sure.
If the process is exiting, then using the Service Watchdog package to monitor it will help treat the symptom, but not cure the original problem.