When a mailbox database in a Database Availability Group environment enters a FailedAndSuspended state, you will be needing to run one of these commands!
The Resume-MailboxDatabaseCopy will attempt to replicate only uncopied transaction logs to bring the database back into a healthy state. Minimal bandwidth utilization and effort required by the servers.
The Update-MailboxDatabaseCopy command on the other hand will perform a complete database reseed between the servers to bring the database back into a consistent state.
As a rule of thumb, you always want to try the Resume-MailboxDatabase cmdlet before you attempt a full reseed.
Be very careful when running the Update-MailboxDatabaseCopy cmdlet if your servers are in different datacenters connected by slow WAN links. If this is the case and you need to run this command, you want to schedule it for a time which will least impact users. Remember, Exchange DAG's do not support bandwidth throttling even in Exchange 2016 primarily because Microsoft thinks everyone should run DAG Replication on a separate dedicated isolated network.