Western digital SSD drive doesn’t mount after upgrade to macOS Monterey.

So you were running Big Sur on this M1 iMac and the WD SSD was Ok?

And the problem with the SSD only appeared after upgrading from Big Sur to Monterey?

If so, I'm guessing you are using the WD software to manage the WD SSD? If that assumption is correct, then you need to install the latest version of the WD proprietary software. To prevent this issue in the future, it is better to let macOS manage the drive instead of relying on third party proprietary software which can break with a macOS update/upgrade.

However, to let macOS manage the drive requires erasing the whole physical drive. To do this now would require you to temporarily transfer all the data to another location because erasing the drive will destroy all data on the drive. How you erase the drive will depend on how you want to use the drive. It would need to be erased as GUID partition and one of the following file systems depending on how the drive is to be used:

• MacOS Extended (Journaled) --- aka HFS+ --- For a data drive for use only with macOS. Also compatible with older versions of macOS.
• APFS (top option) --- newest Apple file system which in theory should be better with less risk of corruption when a drive is accidentally disconnected, but in reality the APFS file system can sometimes become corrupted where First Aid is unable to repair. If this happens, then completely erasing the drive is the only option to fix the file system.
• exFAT --- choose this if you need to share the drive with Windows

If you don't have the proprietary WD software installed (or any third party NTFS driver) and you are letting macOS manage the drive, then disconnect all other devices from the iMac in case one of them is causing a problem. I have seen reports where a connected Wacom tablet can cause problems with some external drives. It is possible other devices could also cause a problem as well.

Is it an internal or external SSD and what troubleshooting have you attempted. Please describe in detail so we have a starting point.