TL;DR Delete the files for the Profiles, Sources, and Scenes.
The Devs for some godforsaken reason cant fathom their code is dogshit and causes arbitrary incorrect behaviour.
When you create a profile (one is created by default called "Untitled" on first start up.) it saves a bunch of files (JSON and bak files) for Scene and Source behaviour.
Sometimes (for literally no apparent reason) you wont be able to see sources/scenes you previously had working on the preview window. You can see them working in the Studio Mode "Program: Scene" Window" and the "Properties for [Source]" preview Window, but for whatever reason, that video feed wont display (no matter how many checkboxes and settings the devs tell you to click) in the Main OBS Window Preview.
Im giving a general answer to the issue that SHOULD get your OBS back to how it worked when you first started using it.
Go to C: > Users > [Your User] > Appdata > Roaming > obs-studio
In here you will have several folders.
Go into Basic and delete both profiles and scenes folders.