[DNSB]

You may want to try a manual factory reset on your Clara HD if you haven't already done so. If that doesn't work, you can ask for an image to try re-imaging your Clara HD's µSD card in the Kobo µSD Card Image Requests thread.

1. Connect your eReader to a power source by doing one of the following:

• Turn on your computer and connect the included USB power cord to your computer first, and then your eReader.
• Plug the wall charger (not included) to a power outlet, and then connect your eReader to the wall charger.
• Charge for 1 hour.

1. Press and hold the power button until you see the words “Powered off” on the screen and until the power light turns off. If there is no response after 30 seconds of holding the power button, continue to the next step.
2. Release the power button.
3. Hold down the power button.
4. The power light will remain on.
5. Once the power light turns off, release the power button.
6. Your eReader will start the factory reset process. You should see a black screen that tells you the eReader is resetting. Do not turn off your eReader during this process.

As for Jon's comment, I suspect he was referring to starting calibre in safe mode not Sigil.

Edit: PMs sent