First, you need to find which mailbox was quarantined. Open Exchange management shell and run the following code:
Get-Mailbox –Server SERVERNAMEHERE –ResultSize unlimited | Get-MailboxStatistics | Where-Object { $_.IsQuarantined –eq $True } | Select DisplayName
Will return the End Users Account whose mailbox has been quarantined
To remove the mailbox from quarantine run the following command
Disable-MailboxQuarantine "UserID"
(so Disable-MailboxQuarantine <MailboxName>)

Now that the mailbox is out of quarantine we know that it may not be in a healthy state because it got quarantined for a reason so you can run the following mailbox to get rid of all the bad bugs and return it to a clean state.
New-MailboxRepairRequest -Mailbox testuser@domain.com -CorruptionType ProvisionedFolder,SearchFolder,AggregateCounts,Folderview
When this request is running mailbox access for this specific person is disrupted because it’s being repaired. After the repair request has been sent it cannot be stopped and in order to stop it, we have to dismount the database which is enough to make us squirm.