Encrypted external HD cannot be mounted

If you cannot resume the encryption process, then you will most likely need to erase the drive & start over. I know when Filevault is encrypting or decrypting a drive that any lower level utilities such as First Aid or data recovery apps will not work since the drive is between two states.

FYI, as you have just learned it is very important to make sure the laptop is connected to the power adapter during any major events, especially events which can take a while to complete. Some events cannot be interrupted.