In OpenERP, deactivating a record hides it completely from the system. However, the record still exists in the database.

To solve your problem, go to your list of users and apply the following advanced filter:
Active is true OR Active is false (see screenshot below)
This will display all the records whether they are active or inactive. You will then be able to get into the record and reactivate it again.
This works for any object in OpenERP.