Valves DW8

Hi Due to a problem in relation to starting on a DW8 engine 1.9 i was advised by a French Auto outlet that this engine is renown for a problem related to valves. My engine started to misbehave over a period of 4 months until it reached the stage of not starting. We removed the Cam cover and tested the gap in the valves to find that four of the valves had no gap at all. We that is my son and I are wondering how the gaps could have closed in this way. It is at this point worth mentioning that the engine turn over very easily, which it did not do before in fact one had to be careful not to run the battery down, as my son had the habit of not waiting for the heater plugs to finish their cycle.

Can anyone explain how these gaps could have closed as I have the awful thought that the valves must have dug into the valve seats.