First find in which database this mailbox exists. If you remember only name of the user, you can use -match operator:
Get-MailboxDatabase|Get-MailboxStatistics|where {$_.displayname -match "Kowalski"}|fl displayname,mailboxguid
Then you should get MailboxGuid you need.
Next, select or create mail disabled user and point it with connect-mailbox command
Connect-Mailbox MailboxGuidhere -database databasename -user newuser
