This is usually caused by either incorrect permissions on the xxxxxxx.num file, or a corrupted xxxxxxx.num file. Firstly connect via FTP to your site, and check/change permissions on file. If this doesn't work, then simply delete the file and Actinic will create a new one.
You can then restore the order numbers in 'Help | Troubleshooting'.
You can then restore the order numbers in 'Help | Troubleshooting'.