VPOP3 version 3 and later use the PostgreSQL] database server. This is an open source enterprise level database server. Several VPOP3 problems are due to problems with the database service.
If VPOP3 won't start even though the process is running, then that is often because the PostgreSQL service is not running. If the PostgreSQL service won't start, or keeps crashing, then it is probably because the database is corrupted. In VPOP3 versions 3 and 4, the PostgreSQL service is usually called 'Postgres-VPOP3'. In version 5 and later, the service is called 'VPOP3DB'
Our chargeable support service will be able to help with this if necessary. Note that we cannot guarantee recovering data, as that will depend on how badly the database has been damaged, and when the latest good backup was.