if as you say you have some of your drive recorded just fine, then i don't think it is the memory card that have failed ( don't think those can fail partly ) cards failing for me over the years have often gone into read only mode.
Meaning you can only copy stuff that is on the card but not put anything new on it or format it.
But if your clock start with the same time every time you start your car, then when you do that what ever you recorded before you stopped get written over as it is then the oldest footage on the memory card.
So say your default time of the camera are 0, and you then start your drive, then files get named 1 - 2 - 3 - 4 as they are recorded ( simplified but a progressing string of time/date filenames ) and so on after that, but if you then stop and start again and the clock reset to 0 then the file named 1 from the session you just drove will get deleted as it is the oldest file.
But then again if the memory card are not full it should not delete any old files to make room for new ones, so i am a bit confused on this matter.
How large is the memory card you are using ?