Perhaps open it in safe mode (hold down shift key while selecting file name). This will allow you to open the project and elect to not instantiate each plugin one at a time thus being able to load your project and insert the proper plugins.
If the project was created in 32 bit Sonar it will have 32 bit plugins. When you try to open the project in Sonar x64, those 32 bit plugins may not be available. Sonar won't automatically insert 64 bit plugins in place of the 32 bit ones. Each plugin is a specific file with 32 and 64 bit files located in seperate places.
If the 64 bit Sonar has recently been installed you may need to reset and rescan all your plugins to make sure that Sonar also sees your 32 bit plugins. Do this first and perhaps the project will load.