Internal HD not mounted after upgrade: Encrypted: ERROR -69808 I am asking for help on getting back into my MacBook Pro. I finally allowed a software update. I was a bit careless and didn't check what was updating. The account refuses my password, so I hold down command-R to boot in recovery mode. 

I open a terminal and if I type: 

diskutil APFS list


I get the following output (abbreviated):

Volume disk2s1 
---------------------
APFS Volume Disk (Role):   disk2s1 (No specific role)
Name:                      Mackintosh HD (Case-insensitive)
Mount Point:               Not Mounted
Capacity Consumed:         73577095168 B (73.6 GB)
Encrypted:                 ERROR -69808

Attempting to repair fails:

diskutil repairvolume disk2s1
Started file system repair on disk2s1 Macintosh HD
Repairing file system
Volume is already unmounted
Performing fsck_apfs -y -x /dev/rdisk2s1
error: mount_apfs exit status 73
File system check exit code is 78
Restoring the original state found as unmounted
Error: -69845: File system verify or repair failed
Underlying error: 78

Running fsck_apfs -n -d /dev/rdisk2s1 gives 
mount_apfs: mount: Input/output error
error: mount_apfs exit status 73


Any ideas how to recover the data?
Even though you have now wiped the drive and are using your MacBook, I would strongly advise that you look at the SMART reports for your drive, especially if it is a traditional spinning HDD — mount: Input/output error suggests that your drive is failing.

For future reference, if your drive is failing, a tool like ddrescue will allow you to create a byte-for-byte copy of the readable data on the drive to another drive. The copy can then be inspected further to attempt data recovery without any risk of complete data loss due to the failing drive becoming completely inoperable.