Sierra stuck at login screen - password doesn't work
Try to reset your login password at first:

macOS Sierra: Reset a login password.

If the above didn't help, try to reset Nonvolatile Random-Access Memory and System Management Controller:

How to reset NVRAM or PRAM on your Mac.

How to reset the SMC on your Mac.

P.S. I experienced the same issue on Mac Mini with El Capital. Resetting PRAM and SMC helped me to fix it.