A luser just ran that hazardous process or spreading on the production server and as a result trashed loads of data on your beloved server.
You cannot afford to take the server down to get yesterday's backup and they need the data now...
Fear not, the transaction log is here to save the day.

• In server explorer, right click on server->View Transaction Log
• Narrow the query as much as you can to the time/client/cube/measures that you are after
• /!\ Mind the date is in north-american format mm/dd/yyyy
• Edit->Select All
• Edit->Back Out will rollback the selected entries

Alternatively, you could get the most recent backup of the corresponding .cub of the "damaged" cube:

• In server explorer: right-click->unload cube
• Overwrite the .cub with the backed up .cub
• Reload the cube from server explorer by opening any view from it