There is a long standing, known issue with save games failing to load after some amount of playing… in the current state there is nothing you can do short of restarting at an earlier point in the game to work aroundit, nor will there be a fix for this issue until the final game is delivered according to the developer.
War Horse have assured us that the save/load system has been completely rewritten, but they will not release any patches to this thing they call a “beta”, which is more like an early alpha/ tech demo in reality. They have also not announced a release date. possibly because they’ve already missed the initial release date by what …about 2 years at this point?