MacBook pro stuck in a loop after booting
It sounds like the OS might corrupt hence failing to boot, you can verify this by booting up using verbose mode to see where it gets stuck at. 

To fix the actual issue I think you should boot into recovery mode and do a re-install of the OS (formally called Archive install) 
This shouldn't delete any data but if you had a backup then good. Here are instructions

https://support.apple.com/en-gb/HT204904