How do I fix this Samsung galaxy S5 glitch ?

I was using my finger print scanner for about 16 months now on my S5, and now, out of the blue it doesn't let me, all it asks for is the alternative password and I have no clue what it is, the strange thing about it, is when I type in the wrong password about 5 times it says "The finger print entered did not match"...
What???

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