If you have just upgraded to OS X Yosemite and are having issues with the application crashing on start, you will have to re-install Injoice.
First, make a backup of your Injoice data and preference files. Have a look at the backup instructions to learn how to do this.
Then, uninstall Injoice. Open up the Launchpad, look for Injoice, click and hold on the Injoice icon until it jiggles. An "X" button should appear; click on that button to uninstall the application.
Once the application has been uninstalled, re-install the application from the Mac App Store, then re-launch Injoice. Injoice should no longer be crashing. If Injoice successfully launches but your invoices and preferences are missing, then shut down Injoice completely (you can press the Command-Q keyboard shortcut to achieve this). Restore the backup files you made earlier, then re-launch Injoice. Your invoices and preferences should be restored.
If you are still having issues, please send me an email with the screenshot of the dialog window with the crash details. Your assistance is greatly appreciated.