One possibility that I just ran into is that you've added a device to your system that causes other "auto mapped" devices to bump up over the mapped drive.
Say you have M mapped to a network drive, but you add a new harddrive, that then causes all the removable drives (even though they don't have anything inserted into them) to bump up a letter (or two).
Now, suddenly, one of the removeable drives is mapped to M.
Your network drive can no longer be mapped to M, and when you try to remove the mapping, it'll give you a "Device already in use" error.
Open up "Storage/Disk management" and check ALL the drives for any mapping that might be on top of the drive you want to map.
If there, just remap that drive to something else.
Then you should be able to use NET USE to attach to your network drive properly.