NVIDIA Driver 416.16 (and above) can cause recording and streaming problems with all products using Elgato Game Capture HD software.
This driver was released on October 4, 2018. If you can't record or stream, then this driver update may be the cause of the problem. NVIDIA Driver 416.34 may also have this issue.
To solve the problem, please try this beta software:
Game Capture HD Beta - 18.104.22.16824 (Windows 10)
There will be a final version of this software soon, after some additional testing is completed.
If you still need assistance, feel free to contact Elgato Support via https://help.elgato.com.