My computer turned off while banshee was updating my music library. Now I can't open it. The error says: "found a fatal error. Exception has been thrown by the target of an invocation."
I'm not sure if this will help, but you could try deleting the local configuration files for Banshee and trying again.
Open Nautilus and press CTRL+H to show hidden files and folders. Browse to the folder .local and inside you will find a folder called share. Open this folder and find the folder called banshee and delete it. Then try reopening Banshee.
If that doesn't work you can try reinstalling the application by using the following terminal commands.
Again, I'm not sure if this will help or not, but I'll add that if you try opening Banshee from the Terminal it should spit out an error report. If you post that here in your question, perhaps someone who knows more than me can help you.