Stolen iPad

Did you not have Find my iPad activated on your device?

See this link on what to do if your device is lost or stolen: If your iPhone, iPad, or iPod touch is lost or stolen - Apple Support

I honestly don't know how that is possible. The sad reality is most lost or stolen iPad's are never recovered anyway. Did you use a passcode on the device? Could anyone you know have gotten into your iPad who may have had knowledge of any of your passwords?

I doubt it. But I'd want to know how it's possible someone's iPad with Find my iPad enabled could have it's iCloud password changed. You didn't answer if you had a passcode on the device.

The only thing I can think of is they knew your email address if that's what you used for your Apple ID.

I have no clue how this is even remotely possible. I'd speak with Apple. It's not likely going to get your iPad back though.