My Mac forgot my password. PEBKAC issue?

Look I'm not going mad, but I swear my Mac forgot my password. I've used this password since setting it up 3 months ago. I type it in 10 times a day. This morning when I got to my Mac it was switched off (odd because I left it running last night) and when I tried to login it complained about my password. I had to go through the whole apple account reset procedure. Bit of a faff. So am I going mad? Or was this a PEBKAC issue and I'm delusional?