Any time an app or utility writes to a drive there is always a chance of something going catastrophically wrong. While it is rare, the possibility always exists and the risk increases if there are any hardware issues (drive failure, bad cable, bad port, bad enclosure, bad adapter/dongle/dock/hub, etc.).
Also if you did not have a backup the first two steps should have been to check the health of the drive. If the drive is healthy, then the next step would be to attempt to clone the drive using a low level bit for bit cloning utility so that you rescue as much information from the drive as possible. Then you need to work from the cloned copy. If the original drive has a hardware failure, then you need to make another clone of the clone since the original failing drive has likely become completely unusable after the first clone. Having a second clone gives you the ability to start over on "fixing" the problem in case you make a mistake trying to "fix" or recover the data. It is a very complicated and time consuming process to do this which is why having backups made before problems occur is so important. If the drive has a hardware failure, then contacting a professional data recovery service would be recommended since you usually only get one chance at recovering data from a failing drive.
My first guess is that the external drive has some sort of hardware failure. Try running DriveDx to check the health of the external drive and posting the report here using the "Additional Text" icon which looks like a piece of paper. You will need to install a special USB driver so that the external drive's health information can be accessed. However, even with the special USB driver you may still not be able to access the external drive's health information since the USB controller on some external drives won't allow access to the drive's health information. Until you confirm the drive is healthy I would refrain from using the drive or even powering on the drive except to retrieve the drive's health report. The more a failing drive is used (or even just powered on) the more likely the failure will just get worse.
Other possibilities are a bad USB Cable or an bad external enclosure. Make sure to connect the drive directly to the laptop. If you have been connecting the drive directly to the laptop, then try using a powered USB3 hub (use a good quality and well respected brand). Also disconnect all other external devices and try using another USB port. If your laptop uses USB-C ports, then try using the ports on the other side of the laptop if you have the four port model.
If you are using an M1 Mac, then perhaps this external USB drive just isn't compatible with the M1 Mac. There have been a lot of posts on these forums about compatibility issues with some USB drives and M1 Macs. If you are using an M1 Mac, then try connecting the USB drive to an Intel Mac to see if you can see its contents.
What file system is used on the external drive? It is best to use HFS+ (aka MacOS Extended (Journaled) ) whenever possible since APFS is a new file system that seems to be become corrupted and First Aid is unable to repair it requiring a complete erase.
Is this external drive a hard drive or an SSD? If it is an SSD, then most likely the data is permanently lost if you don't have a backup. Be prepared for this possibility.
If you don't have a backup, then you may want to contact a professional data recovery service such as Drive Savers or Ontrack. Both vendors provide free estimates and both are recommended by Apple. Keep in mind the more you do to the drive the more unlikely that even a professional data recovery service will be able to recover any data from the drive even if the drive is healthy. You may only get one chance at recovering the data so choose wisely.
FYI, you should always have frequent and regular backups of your computer(s) and any external media which contain important and unique data. The more important the data the more copies or backups of the data you should have.
