You probably have a "special" character in the file name/path. This is usually caused by a non-English character in your name or login name for windows, where you store "my documents", e.g. C:\Users\Öleg\My Documents\. Non-English characters, numerals, and symbols cannot be read by Terragen, but they work for your operating system if you have it set to a non-English language, so you're able to save the file name that way, but when Terragen tries to load it, it encounters an error. The solution is to save to a location without non-English characters. If you want to fix the file you are having problems with you can open it in a text editor and search for non-English characters in the paths of files, etc. that are specified.
There are several other threads about this if you need more info.
- Oshyan