I need to lock an iPad that was stolen and they know my password

Here is Apple’s advice for lost or stolen devices:

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

Use the Find My app to locate a lost or stolen device – Apple Support

If Find My was enabled prior to the device being stolen - and the iPad has an active network connection - you should be able to lock the iPad remotely. If instead you had not enabled Find My, you will not be able to remotely manage or disable the stolen device.

In either case, you would be well advised to change your AppleID Password:

Change your Apple ID password - Apple Support