Old or new computer/OS?
even if old, with driver and OS updates, what OBS is pointing to technically may not exist anymore, and you may need to re-select Video source
If new system, I always recommend making sure camera works outside of OBS first (in a native OS app). Then make sure OS permissions allow OBS to use camera, then setup camera in OBS
As always, don't ignore the pinned post in this forum regarding posting your OBS log so we can review your OBS settings to see if anything obvious sticks out
