Some features like "Save as", Particle options and Exporting the animation as sprite sheets are in grey, meaning not available. They were available before but now I can't seem to make them work.
There are a couple of reasons why this is happening.
The application doesn't have necessary rights to save the files. It has been started under a Limited user. Run it as Administrator to overcome this issue.
The trial version has expired and it doesn't allow you to use these features anymore.
The software is corrupted and the registry entries are not in a good condition. Re-installing should fix this as well. You can obtain a download link through the official website.