The first thing to do is reboot the device. Detailed information is available from CNET.com: http://www.cnet.com/how-to/how-to-fix-95-percent-of-ios-app-crashes/
If this doesn't help, there are two common possibilities: the first is a damaged file or a file that was not created or last edited in Final Draft 10 / 9 / 8 or the Final Draft Writer. The second is a damaged installation of the app itself.
Some other screenwriting programs can save as FDX, but these files are not written in genuine Final Draft XML and the scripts may not look or behave as expected when opened in Final Draft 10 / 9 / 8 or Final Draft mobile apps
The remedy is to open the script in Final Draft 10 /9 / 8, hit the spacebar, hit backspace, save it and send it to the iPad again. If this doesn't work, see this article for information on how to rebuild the file: How do I rebuild a corrupted script in Final Draft 10 / 9?
If the app continues to crash even with files known to be healthy, see this article for information on how to uninstall and reinstall it: How do I uninstall and reinstall the Final Draft Writer for iPad?