Joomla 3.6.3 improved validation of JSON data stored in the database (Usually as params for extensions). Unfortunately, this means that after updating, sites with invalid data can can become inaccessible.
The usual error message shown is:
0 - Error decoding JSON data: Syntax error
So a nice script is developed by Rob Went to repair the syntax errors and you can review the description and download the script here here on Github.
Thanks to Rob Went for the great script and clear instructions!