My MacBook keeps saying that my password is wrong, even if i am 100% sure about my password

My MacBook keeps saying that my password is wrong, even if i am 100% sure about my password. Then now i says like this when i tried to forgot my password. What should i do? What is the problem of this? How can i fix this issue? If a reset it the macbook it would be back from the start? Please help me in this problem.