I ran into this problem tonight. Here's what I did to fix:
Go to Device Manager
Scroll down to the Universal Bus Controller section
Are there any warning signs next to any item? If so, right click on it and uninstall.
Create an example scene with Webcam
Create a new scene and call it Example
Add a Video Capture Device
Right click on it to get to the Properties window
Select your webcamera as the device
You should see in the preview window its real time captured image to confirm it is working
Click the OK button
You should see it appear in the preview of the scene
If you don't see the webcam immediately appear even though it is the default device, it isn't working because it hasn't been initialized with your selection. It looks like it is the default device. I had a case where it happened to be the default selected device, but however nothing came up in the preview screen.
After I reselected it, I got a OBS crashed. Then, it magically appeared. Definitely acting flaky.
I also noticed that if I already have an existing source for the webcam and renamed it from video capture device to "webcam", choosing it as the source does nothing. I always had to create a new video capture device, never rename it. Selecting a previous instance of a web camera device never worked and ended up with a blank screen with no component showing up with handles.