Using High Sierra, Filevault is stuck x 5 days - changed macintosh HD startup disk to external driv e

I have rebooted in recovery mode, > disk utility > macintosh HD not available to turn off encryption. External guide, "Filevault not turned on". Macintosh HD shows not mounted - APFS (encrypted).

How can I access the stuck volume on macintosh HD so that I can turn off filevault encryption.

I used external drive to back up my MacBook pro.

Can I use Time Machine to go back to time before initiating file vault - will it undo the encryption and re-establish macintosh HD as my startup drive and get me back to where I was before the encryption started

Thanks,