Unlock it, recycle it, it’s yours.
You can’t just unlock a blocked phone. Networks can get them unblocked though.
You can get the IMEI changed, but that is hard with modern phones and really quite illegal as it’s something that only people who nick phones would ever want to do. Plus if it is a live IMEI that is cloned, you could get in trouble, as networks can detect that a phone is in two places at once / using two sims at once.
I’d power it up and look through the phonebook, someone on there (home, work etc.) will know who has lost their phone. If it wouldn’t power up, I’d stick the sim in my phone (or an old phone) and see if it had any phone numbers on.
Joe