Unlock my iMac

Impossible to say with absolutely no details.

Why not?

How is it locked exactly?

What happens when you try to unlock it?

Provide some background and specifics on your situation and maybe someone can offer some advice.

So you're saying you forgot your password? Or is are you talking about a firmware passcode? Or possibly iCloud lock?

Throw us a bone here...

See this article: https://support.apple.com/en-us/HT202860

In what way is it "locked"?