Lost iPad mini

To find your IMEI, simply sign-in to your AppleID account from any computer using a web-browser:

https://appleid.apple.com

Having signed-in, open the Devices tab. Here you will find an entry for every device associated with your AppleID; tap the related entry to see full technical details of the device - including IMEI/MEID of Cellular devices.

Additional information about finding your IMEI can be found here:

Find the serial number or IMEI on your iPhone, iPad, or iPod touch - Apple Support

As for your iPad being lost or stolen, the following information may be useful:

Locate a device in Find My on iPad - Apple Support

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 the Find My iPad feature was enabled prior to it being “misplaced”, then if active, you would stand a good chance of locating it. 

However, if the feature was not activated prior to being misplaced - or the iPad has been inactive for more than 24 hours, the last known location will not be available - and the opportunity to use technical means to recover the iPad have been lost.

If you have a WiFi Only model iPad, there won’t be an IMEI.

Only WiFi+Cellular models of iPad have cellular hardware - and therefore an IMEI.