If you have a folder or folders missing in Drive it's probably down to that folder being moved by a user. The tip below explains the situation.
The solution is to ask all users with access to that folder to conduct an advanced search, choose "folders" and enter the exact folder name. If the folder still exists it will be found by Google on the users drive, that user needs to "Add" the folder back where it used to be.
If this doesn't solve your issue, please log a ticket with as much detail as possible, including:
- Owner and Url of parent folder
- Owner and Url of missing folder
- Permissions of missing folder
- Log in to a Google Apps account with permission to access either folder