A bug in OBS can cause this to sometimes occur. The best way to resolve the issue is to right click the "Video Capture Device" source (or the unique name it was given) and select properties.

Inside of properties, just below the "Device" drop down box, you should see a "Deactivate button". Click the Deactivate button, and remove the HDMI cable that is connected to the "IN" port on the capture card.

Wait 5 seconds, and then reconnect the HDMI cable from the source system to the "in" port on the capture card. Once reconnected, right click the "Video Capture Device" source (or the unique name it was given) and select properties. Once inside properties, click the "Activate" button and you should be back up and capturing!