I'd check to see if the context class was uploaded properly. It should be in the library folder (I assume you're working with Vanilla 0.9.2.x). If you see the file, open it up to make sure there are contents inside - sometimes ftp programs screw up and only upload the shell file with no contents.

If the file is there and there is code in it, check to make sure that your library path is set up correctly in appg/settings.php