Press Start and type "Disk Management", you'll see an option called "create and format hard disk partitions" appear at the top. Click this option.
You'll see a window called "Disk Management" appear - and you'll see multiple drives listed. If you can remember the rough size of the M drive look under the "capacity" column. When you see the drive that should be 'M' right click and select "Change Drive Letter and Paths".
You'll see "add" and "change" buttons, press change and then find 'M' in the dropdown. OK all boxes and you should see the drive in My Computer as usual