What operating system are you using? The steps to repair may vary greatly depending on this.
If it's windows XP, have you tried restoring to a previous working point? See if that brings the context menu back? It's possible that you installed something that broke it.
If not, it's a bit of a pain sometimes to bring that menu back involving quite a bit of registry editing and lookups. Usually it's relinking several registry entries, and an OS reinstall is a better path than fighting those entries.
Hope this helps..