Macbook pro keeps restaring again and again after installing the recommended Sierra upgrade

Internet/Network Recovery of El Capitan or Later on a Clean Disk

1. Restart the computer. Immediately after the chime hold down the (Command-Option-R) keys until a globe appears.
2. The Utility Menu will appear in from 5-20 minutes. Be patient.
3. Select Disk Utility and click on the Continue button.
4. When Disk Utility loads select the drive (usually, the out-dented entry) from the side list.
5. Click on the Erase tab in Disk Utility's main window. A panel will drop down.
6. Set the partition scheme to GUID.
7. Set the Format type to APFS (SSDs only) or Mac OS Extended (Journaled.)
8. Click on the Apply button, then click on the Done button when it activates.
9. Quit Disk Utility and return to the Utility Menu.
10. Select Reinstall OS X and click on the Continue button.

Note:

1. To install the version of OS X that was currently installed use Command-Option-R.

2. To install the original factory version when the computer was new use Command-Option-Shift-R.

Get an external drive with no data on it. Try booting into the Recovery Partition (command - R on a restart) and select the old drive in Disk Utility, then Edit/Restore and select your Internal Drive in Restore from. Select the new drive as the destination. This will likely reformat the external drive erasing all data. If that won't work, try booting into Internet Recovery per Kappy's instructions, and see if you can copy the data to the new external from there. If successful, that will save your data,

Given the nature of what you have described, it is likely your data is already lost.

How so? Does it start up now?