You can try another healthy Mac to access the SanDisk SSD. If the external drive works on another Mac device, then the first Mac on which the drive files not showing up should take the blame. Otherwise, the problem is caused by the connection or the drive failure.
If the SanDisk SSD works fine on another Mac, you can troubleshoot the issue with these methods when you can't see files on external hard drive on Mac:
If you can't see files on SanDisk SSD on another Mac, the issue could be caused by the drive itself. Given that you have already tried to show hidden files and run First Aid but the issue remains, then the file could be corrupted due to viruses, malware, and so forth.
The last resort you can try is to restore data from it. If any backup is available such as Time Machine, iCloud Drive, etc., you can restore your files on the SanDisk with these backups, otherwise, the professional third-party data recovery software such as iBoysoft Data Recovery for Mac is what you can rely on. Also, you can recover files on the SanDisk SSD with a local repair service but the cost could be expensive.