How do I find my IMEI number of my lost iPhone?

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

If none of those work then there's nothing you can do to get the IMEI.

For other "blocking" options read this support article about what to do if a iPhone or iPad is lost or stolen -->

If your iPhone or iPad is lost or stolen - Apple Support

If you have access to your iCloud you can use the apple support app to get your serial number of the device. But to my knowledge getting your IMEI number would have to be done manually on your phone. Additionally if by “block it” you mean disable the device to make it unusable for whoever stole your device you can disable the device and mark it as stolen in the find my device app and the find my device website.