Then perhaps Windows has assigned a different drive letter in the file system.
This is the most likely occurance with Windows. If, as you say, you can use Windows File Explorer to see this drive and can drill down through your folder structure to images and see them as well, then it is very easy to not notice that the drive letter shown in the LrC Folders Panel for that drive is different than the drive letter shown in File Explorer.
Drive Letter changed (Windows)
Lightroom did not rename your drive, windows did. Before you re-started Windows the last time you unplugged or removed some device (like maybe unplugged a card reader or took out an SD card) and due to that what LrC had previously known as the E drive was now known as the D drive.
This happens all the time with Windows unless you assign a permanent drive letter to your External Hard Drives - instructions below.
As long as you have not made the mistake of re-importing removing missing images in response to this issue you just need tell LrC what the new drive letter is. But, this will just happen again and again. A better idea is to tell Windows to always use the same drive letter for this drive whenever it is plugged into this computer (it the drive is used on multiple computers you’ll gave to tell Windows what drive letter to use on each computer).
Pick a new letter for your drive that is at least half way down the alphabet (most people use "P" for Photography). Then in the LrC folders panel, right click on the highest level folder shown for the 'missing' drive and tell it to locate missing folder. This will present a Windows Explorer window. Navigate to the SAME folder on the newly re-lettered "P" drive and click "choose". All should then be back to normal but here are a few things to watch/check
You may need to also reset the folder where LrC places images synced down from LR/Cloud. This setting is in the “Lightroom Sync” tab of the “Preferences” dialog.
When you next open the import dialog it may complain that some folder is no longer there and it will use the "Pictures" instead. Ignore the message but be sure to select the proper folder on the "P:" drive in the Destination Panel
If you have backup utilities suchs as Backblaze, Goodsync, or any others, make sure you re-point it to the new drive letter.
Here is a link to a Microsoft document telling you how to assign a permanent letter to a drive
https://learn.microsoft.com/en-us/windows-server/storage/disk-management/change-a-drive-letter
Here are the steps outlined in the above link
1) From the Windows
Start menu, search for and open Disk Management with Administrator permissions.
2) Expand the
Storage node and select
Disk Management.
3) You can also get to the
Disk Management screen by
- Open the Control Panel.
- Open Administrative Tools.
- Open Computer Management.
- Open Disk Management.
4) Locate the volume for which you want to manage the assigned drive letter. Right-click the volume and select
Change Drive Letter and Paths.
5) If the
Change Drive Letter and Paths option is missing or not available, see
Troubleshoot volume and drive.
6) Update the drive letter assignment:
7) To change the existing drive letter assignment, select
Change.
8) To add a drive letter for a new drive, select
Add.
9) Select the
Assign the following drive letter option. Expand the dropdown list, select the drive letter to apply, and select
OK.
10) In the confirmation dialog, select
Yes and complete the assignment.
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