Can someone active an ipad after reporting lost?

Providing that you had activated the Find My service on your iPad, prior to loss, the iPad cannot be activated by anyone else. The associated Activation Lock cannot be removed by anyone other than you - as only you have the AppleID credentials that prevent reuse.

As for your lost iPad, see:

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