That message indicates that CostGuard is not pointing to valid data files. So either:
1. The setting that points to the data path has changed.
2. Your data files changed location. This can happen when you move from a standalone to a server.
Please check that your data path is correct by running the Config program: Program Files(x86)/CostGuard/Config.exe
If you have a server and workstation setup you will need to do this from the workstation that displays the error.
If the path looks good then check if you are able to explore to the data file.
Once you are in CostGuard Shared\Data\ try to create a new file. This will allow you to see if you have Rights. Sometimes those Rights change and suddenly CostGuard no longer has access to the data file.
The user needs read and write Rights to the data on the server as this is where CostGuard looks for the data Please see this information on changing rights.
One other potential issue is your anti-virus software. Please make sure that CostGuard is whitelisted/excluded so that your anti-virus software does not block access to the data file.
And finally, you may need to do a reinstall following these directions for administrator install.