Sorry about your experience, you can try these tips to test whether your external drive can be used again:
- Reset the PRAM/NVRAM on your Mac, read: What is NVRAM/PRAM, and how to reset?
- Restart your Mac in Safe Mode, read: How to Boot Your Mac in Safe mode?
- Reset your exFAT hard drive permissions: Open Finder > Application > Terminal, then type sudo chmod -R 777 /Volumes/YourDriveName (YourDriveName is your exFAT drive name)
- Assuming these ways below won't help you, maybe you have to backup and format the drive. You can use iBoysoft Data Recovery for Mac to recover all your files then do some formatting.
Hope you can access your exFAT drive on your mac again.