What sort of files are contained in the folder that you’re trying to open?
There are global settings that control files to not display in the side bar (shown below with default values), so if the folder you’re trying to open contains only files of those kinds, that would be one explanation.
// folder_exclude_patterns and file_exclude_patterns control which files
// are listed in folders on the side bar. These can also be set on a per-
// project basis.
"folder_exclude_patterns": [".svn", ".git", ".hg", "CVS"],
"file_exclude_patterns": ["*.pyc", "*.pyo", "*.exe", "*.dll", "*.obj","*.o", "*.a", "*.lib", "*.so", "*.dylib", "*.ncb", "*.sdf", "*.suo", "*.pdb", "*.idb", ".DS_Store", "*.class", "*.psd", "*.db", "*.sublime-workspace"],
Failing that, do you see any messages or errors in the console (View > Show Console)?