iPhone stolen

If your iPhone, iPad, or iPod touch is lost or stolen - Apple Support

whoever stole your phone changed the passcode? That would imply that they knew your passcode..

Or is the phone just disabled/unavailable due to them trying to guess your passcode?

If you forgot your iPhone passcode - Apple Support

How to remove your iPhone, iPad, or iPod touch passcode without a computer - Apple Support

The only person who can change the passcode is the person who originally set up the phone. That would be you...... unless......you have given your Apple ID and Password to someone else, and that someone else was also the person who stole the phone.

Work your way carefully through this Apple Support document:

If your iPhone, iPad, or iPod touch is lost or stolen - Apple Support