MacBook Pro 8,1 will not boot to OS, shows Instructions to shutdown and restart on Kernel screen...

Try the following: SMC and NVRAM resets:

https://support.apple.com/en-us/HT201295

https://support.apple.com/en-us/HT204063

Then try a safe boot:

https://support.apple.com/en-us/HT201262

Any success?

Ciao.

The startup drive is failing, or there is some other internal hardware fault.

Back up all data on the drive immediately if you don't already have a current backup. There are ways to back up a computer that isn't fully functional—ask if you need guidance.

Make a "Genius" appointment at an Apple Store, or go to another authorized service provider.

If privacy is a concern, erase the data partition(s) with the option to write zeros* (do this only if you have at least two complete, independent backups, and you know how to restore to an empty drive from any of them.) Don’t erase the recovery partition, if present.

Keeping your confidential data secure during hardware repair

Apple also recommends that you deauthorize a device in the iTunes Store before having it serviced.

*An SSD doesn't need to be zeroed.

Make an appointment at an Apple store genius bar for a FREE evaluation. Warranty status is not relevant.

Ciao.

You may take out the HDD and you might be able to copy your data off of it to another drive if you do not gave backups.

Linc Davis wrote:

The startup drive is failing, or there is some other internal hardware fault.

Back up all data on the drive immediately if you don't already have a current backup. There are ways to back up a computer that isn't fully functional—ask if you need guidance.

Make a "Genius" appointment at an Apple Store, or go to another authorized service provider.

If privacy is a concern, erase the data partition(s) with the option to write zeros* (do this only if you have at least two complete, independent backups, and you know how to restore to an empty drive from any of them.) Don’t erase the recovery partition, if present.

Keeping your confidential data secure during hardware repair

Apple also recommends that you deauthorize a device in the iTunes Store before having it serviced.

*An SSD doesn't need to be zeroed.

Don't you think that it will be difficult to perform some of the activities you have suggested since the MBP will not boot?

Ciao.

The OP can take the back up by the means of Target Disk mode by avoiding the fuss of taking out the drive and searching for an enclosure for the hard disk to take the backup.

Simply by connecting to another mac if possible...!!

Grane Duke wrote:

The OP can take the back up by the means of Target Disk mode by avoiding the fuss of taking out the drive and searching for an enclosure for the hard disk to take the backup.

Simply by connecting to another mac if possible...!!

Yes, that certainly is a possible option assuming that the MBP will boot onto Target Disk mode, another Mac is available and the appropriate connecting wire is at hand.

Ciao.