Disk encrypted with Veracrypt full disk encryption suddenly gone undetected

Before opening your encrypted disk you will have to make it reappear. The encryption process is just reading and writing encrypted sectors and does not cause your drive to disappear.

Your description suggests a hardware failure during that process. This is your first problem. You should connect your drive to other machines if it does not appear there as well. If that's the case you have a hardware issue with the drive. That could range from a failing power supply to the drive itself.

But even if you get that drive back running I don't know if Veracrypt remembered where the encrypted sectors ended and where the unencrypted sectors start. That is the second problem.

I know the Truecrypt documentation and the Veracrypt documentation is derived from it. I do not remember any information in there for your second problem. You might have to try that out after duplicating your currently failed drive.