Your antivirus may have detected some files in the download as malware and deleted them.
Try disabling your antivirus and downloading Storyline again.
It's possible that your PC's RAM is being exceeded by the Modpack's requirements.
You can try lowering the game graphics to "LOW" from the "Graphics" section in the Launcher:
Install Visual C++, DirectX 9 and the .dll libraries from this link (Restart your PC after installation.)
Alternatively, you can disable "ASI Plugins" from the "Compatibility" section in Luancher:
Restart the mission and see if the problem persists.
If the problem persists or is very common, you can disable the "Cosmetic Plugins" from the "Compatibility" section in Luancher:
Try downloading the Lite Version of Storyline.
Or as a last resort, you can activate "LITE Mode" from the "Compatibility" section in Luancher: