Once I was working with a Report Designer and suddenly had electricity cut off. When I launched my PC after the electricity cut off I faced the error “Root element is missing”.

After small investigation I found next solution for this issue

Step 1: Close all Report Designer instances.
Step 2: On your computer, navigate to the Roaming folder located in the %AppData% directory, for example: C:\Users\%userprofile%\AppData\Roaming.

Step 3: Delete the config.xml file.

Then everything should works fine