Exchange 2013 Cannot open mailbox microsoft system attendant

While doing some Work on my Exchange 2013 server, a, for me, new error popped up:

For some reason it turned out my maildatabses weren’t mounted.

See if your database/s are mounted. Important to remember the “-status” or the column will be blank.

Get-MailboxDatabase -status |select name,mounted

Mount all your databases and check status.

Get-MailboxDatabase | Mount-MailboxDatabase
Get-MailboxDatabase -status |select name,mounted