In the comments there is information that his plane was shot down near Tver. This is a slight surprise as he had just published a video purporting to be recorded in Africa.
Tver is just NW of Moscow, almost on a line drawn between there and St Petersburg. Over the past week there have been almost daily drone attacks on Moscow and the Russians reported downing one on its way to St Petersburg. There are also Russian airbases dotted around Moscow.
[My thoughts]
Prigozhin has often had the transponder on his private plane turned off for operational security reasons. So the cause could be accidental targeting by Moscow air defense when they detected the plane on radar but no “identify friend or foe” information.
Of course the downing of Prigozhin’s plane and the apparently bad health of Igor Girkin may have more than temporal coincidence.