My cellphone was stolen

The thief would not have been able to disable Find My unless he had the passcode for your phone. If he/she does not have the passcode, the phone is a "brick".

If the phone does not show up on Find My, the phone is likely powered off or the battery has run down.

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

Try log on to iCloud.com, enter your Apple ID, etc, click on iPhone, then click on All Devices.

Click on your Phone

See if you can enable Lost Mode on the phone

This will lock the phone so that the data cannot be accessed

Lot of details here.

Use Lost Mode in Find My iPhone on iCloud.com - Apple Support