What is your hardware and which screen do you use for the program and the same for preview?

CPU: Ryzen 5 2600X

GPU: GTX 1650 Super

RAM: 16gb DDR4

It was working fine the first time I loaded the program, but every time since it hasn't. I've tried reinstalling. And I do use the same screen for the program and preview.

Try resetting exactly according to instructions. (It's not the same as reinstalling.)

Reset Vegas 18 as follows:
Close VEGAS Pro
Press CTRL+SHIFT on your keyboard while double-clicking the program's shortcut icon on the desktop to launch it *.
Continue holding CTRL+SHIFT down until you see a window that asks "Do you want to reset all preferences to default values?"
Place a checkmark next to "Delete all cached application data" and then click "Yes".

After that download and install the files you see to download (if needed) at
VPro 18/Help/Check for Driver Updates and after that do't change anything.
Try again to view the Preview.

@vkmast 😉😉😉

Try resetting exactly according to instructions. (It's not the same as reinstalling.)

Resetting (including deleting cached application data) did not resolve the issue. It still stays a blank screen. Here's a .mp4

https://www.gravityisjustatheory.com/sharingiscaring/MW5gCdkw1Q.mp4

It is not specific to the video file either, any visual media does not load in the viewer, but renders fine.

@aaaa It's possible to have the Preview screen black if split screen is selected, "Select All option", and also "select clipboard". But your mp4 doesn't appear to show it as selected.