I am not a computer expert here but if a program crashes and you cannot restart it, parts of the program may still run. Or at least the pc thinks it is running and won't open it twice.
If that happens again try opening the task manager (right click on your taskbar or ctrl+alt+del) and look at "processes" if the program is still mentioned in that list. If so: Force it to stop running ("End Task" it is called in English I think .... Sorry, my windows is in German so names may differ).
Anyways after that is done, it might be possible to restart Clip Studio ...
Anyhow if those crashes happen regularly that can happen several issues:
- Another programm might interefere (like Virtual Keyboard does with Skyrim)
- Clip Studio isn't up to date anymore and you might want a newer version compatible to your system
- It might have bugs (same thing, get a newer version if possible)
- The progam is damaged (this can happen if there are PC-Errors. It isn't needed that those errors appear with a warning that flashes up. Irregular shut downs. Energy fluctuations in the system etc.) In that case: Reinstall like @elyss adviced.
Ther may be more causes but like said: I am no IT expert. ^^