1) Make sure there is an exception for PSO in your antivirus.
2) Make sure you did not select an display resolution higher than your monitor.
3) Run the game from the launcher again and let it patch completely.
4) Cross your fingers?!
This is also happened to me after a few succesful plays. Your windows defender is automatically deleted a file in your folder where you have PSO saved and it does this without any sort of notification. What you need to do is go to your windows defender settings and add the entire folder to the exceptions list that way it will completely ignore that folder. Hope this helps!