Create a new drawer in Devs from within Workbench and rename it to "Monitors".
This won't write over anything if the drawer already exists. To be honest it sounds to me like you've moved the Monitors drawer somewhere else, probably with a directory utility...but it's left the icon where it was (Monitors.info) which it would do unless the drawer and its associated .icon file were moved together. Have a good look around for it, I'm sure it'll be there somewhere. Unless of course you've deleted the Monitors drawer instead...leaving its icon intact. In which case just copy it over manually from the Storage Disk of your Workbench floppy set.
If you do decide to copy it over from the Storage disk, you'll have to delete the old .info file first. This is just a matter of deleting the "Monitors" drawer that isn't opening and then dragging the new one across from the floppy disk.
