Some high-end Samsung phones messed up by Marshmallow update; fingerprints and passwords not recongized
After the Android 6.0 Marshmallow update rolled out, some users began reporting that their devices are neither recognizing their fingerprints, nor accepting their passwords, thus effectively leaving them locked out. The only workaround for now is to have a Samsung account associated with your device, to log with it into findmymobile.samsung.com and to remotely lock and unlock your phone.
Although the issue is affecting only a small proportion of Samsung users, the only sure way to prevent this from happening is to turn off security beforehand and then update to Marshmallow. It's still unknown what causes the issue and why it's affecting only certain users, but we sure hope Samsung looks into it soon.
Source: Reddit via AndroidAndMe