Chosen Solution
Turn on, get log on screen, type in password, log in screen comes back up, start typing password again and keyboard and mouse freeze. I can’t log on!
I suspect your HD has some corruption or the HD SATA cable is failing. Odd behaviors like this often happen when the system tries opening a lot of files at once after an upgrade. The best thing here is to find a second Mac system which you can connect in Target Disk Mode so you can salvage anything important or create a backup. Once done, you can try to reformat the drive from the other system and then reinstall the OS and restore your backup. Mac startup key combinations Let us know how it goes so we can guide you which way to go next.
Start the laptop up in Safe Mode. Hold down “SHIFT” while turning on the computer from an “powered off” state. If you can login in safe mode, Follow Dan’s advise and backup your stuff, then format and reinstall the OS from scratch. This will require a bootable thumb drive installer for Sierra or High Sierra.
