240v 3 prong - no voltage with neutral
The third wire is not connected, either broken somewhere along its path or there's a missing connection in one of the junction box leading to the outlet.

If there's multiple connection boxes from the panel to the outlet, you can test at each to figure where the problem is. Either a nut came loose somewhere or a wire was cut/torn.

The question doesn't make it clear whether you are in a location with 240 and ground, or 240 with neutral and ground, but the answer remains the same, the third wire is disconnected somewhere.