If a Tenant has been vacated in error, you can reverse this action as long as another Client hasn’t been tenanted in to that Room, and the Room status is ‘Vacant Tenantable’.
Open the Tenancy Search Screen, and click  to show all active and vacated records.  
The default for the Tenancy Search screen is to show only Active Tenants, and clicking Show All + Vacated will display all Tenancy Records. It is for this reason that you may see numerous Tenancy Records for one Tenant.

Once in the Vacated Tenancy record, double click on the words 

A warning message will appear advising you that the Tenancy will become Active again. Click Yes to continue. 

Clicking Yes will re-activate the Vacated Tenancy:

The Vacated Date is now blank 

The Tenancy Status shows as 

The  button is enabled

A Tenancy Reactivation Note has been created in the Notes section

The Tenancy Search screen will show the Tenancy Status as Active/Active Arrears