Apple ID Password doesn't work anymore

Hi!

I first noticed an issue with my iMessage on my Macbook not updating anymore. After investigating, I discovered that in the Apple ID section of System Preferences my phone number was no longer associated with my Apple ID. I added it and thought that would fix the iMessage issue. I tried signing out and back into my account in iMessage and it kept saying my password was incorrect. I never changed my password. In fact, under System Preferences it even notes the last time I changed my password was 2016. I tried to change my password from System Preferences and also got an incorrect password message.

What could possibly be going on and how am I supposed to fix it???

Thank you in advance for any help!

-Liam