I just had an issue where a 900f wouldn't turn off after 4 months of being in service. Working perfectly fine for the first 4 months with C4.

Could turn the tv on 100% but it wouldn't turn off anymore. Sounds similar to your problem.

Had to go and update the driver to the newest version to get it working properly again. No idea why it stopped working. It wasn't due to an update because I disable automatic updates.