Forgot my MacBook 2011 Password

I stopped using my 2011 MacBook Pro for a few months, and now that I want to use it it doesn't recognize the password I've always had. In the System Preferences Users & Groups panel I try to enter through Set Apple ID, an ID I have, but then it asks me first for my old password, precisely the one I don't have. I went to Apple Support, but when I type in the MacBook's serial number it tells me they cannot help, I suppose because of its age. What could I do?

Thanks in advance for any input,

Ram