Failover occurred because the target was monitoring the source for a failure, and when a failure occurred, the target stood in for the source. User and application requests that were directed to the failed source are routed to the target.
While the users are accessing their data on the target, you can repair the issue(s) on the source. Before users can access the source again, you will need to restore the data from the target back to the source and perform failback. Failback is the process where the target releases the source identity it assumed during failover. Once failback is complete, user and application requests are no longer routed to the target, but back to the source.
Ideally, you want to restore your data from the target back to the source before you failback. This allows users who are currently accessing their data on the target because of failover to continue accessing their data. Restoration before failback reduces user downtime. The procedure to restore and then failback varies widely with server and network configuration. Another method, which may be easier in some environments, allows you to failback first and then restore the data from the target to the source. A possible disadvantage to this process is that users may experience longer downtime, depending on the amount of data to be restored, because they will be unable to access their data during both the restoration and the failback.