How can I access my old iPhone SE after a passcode lock?

Hello,

I was just wanting some assistance to get back into my old phone iPhone SE. I currently use a iPhone 12 as my current phone.

For Some reason the passcode isn’t working to access it on iPhone SE. There is no doubt about the accuracy of this passcode. The last time I accessed it a few months ago, I had my iCloud account signed into it. Now when I check my iCloud, the backup for this iPhone SE no longer exists and I am no longer signed into this iPhone SE.

What is going on? How do I retrieve my account? What else can I do?

Thankyou.

[Re-Titled by Moderator]