Partition showing as 'FFFFFFFF-FFFF-FFFF-FFFF-FFFFFFFF'; attempted to fix it but made it worse
The disk lost any gpt partition in the partition table because the secondary GPT mismatched the primary GPT (and its CRC was wrong) and thus was bogus.

To recover everything the following was done:


boot to a working external macOS boot disk
get the details:

diskutil list
gpt -rvvv show disk0
fdisk /dev/disk0
diskutil umountDisk0

completely zero out the 1st and 2nd GPT and the MBR:

dd if=/dev/zero of=/dev/disk0 count=34 bs=512
dd if=/dev/zero of=/dev/disk0 seek=(size_of_disk_in_blocks-33) count=33 bs=512

create a new gpt:

gpt create -f /dev/disk0 

add all partitions

gpt add -i 1 -b 40 -s 409600 -t C12A7328-F81F-11D2-BA4B-00A0C93EC93B disk0

a hexdump of block 409640 revealed a CoreStorage partition - a pic taken by the OP before, suggested a size of 187683544 blocks for the second partition:

gpt add -i 2 -b 409640 -s 187683544 -t 53746F72-6167-11AA-AA11-00306543ECAC


the volume verified with 0 errors and had to be unlocked: the volume is encrypted!
Tried to restore a previous recovery partition:

gpt add -i 3 -b 188093184 -s 1269536 -t 426F6F74-0000-11AA-AA11-00306543ECAC
diskutil verifyVolume disk0s3


No file system was detected; the diskutil verifyVolume command failed. The recovery partition and its content was deleted previously. The third partition has to be removed again:

gpt remove -i 3 disk0

Since the recovery partition - usually containing an intermediate FV key required while booting to FV - is missing, the main volume was reverted to HFSJ:

diskutil cs list #get the lvUUID of the main volume
diskutil cs revert lvUUID #old (admin) password needed to unlock the volume

The volume was expanded

diskutil resizeVolume disk0s2 0
diskutil verifyVolume disk0s2

Booted to the restored and expanded main volume the system (Sierra 10.12.6) was reinstalled to restore the missing recovery partition.
Re-enable FileVault.