Hi
Before doing anything else try and make sure you have a current backup.
Have a replacement disk handy. Warn everyone to save their data and to disconnect from any mounted server shares. Shut down the server remove the failing disk and install the replacement disk. Reboot the Server. Launch disk utility and monitor the rebuild of the mirror. If it's not rebuilding click the option to do so.
Depending on how much data you have it can take some time. You should be OK to use the Server whilst its rebuilding but bear in mind performance will suffer in some way whilst its rebuilding.
HTH?
Tony