Sorry if this has been addressed here but I can't find anything specific to our situation. There are plenty of 'missing ribbon' threads but reseting the UI does not fix. I think our issue may be a little unique. It seems that when users are doing work in Adobe creative suite 4 (especially adobe acrobat 9), the ribbon in revit will disappear occasionally. This has been the case for 4 users. I've tried reinstalling revit and CS to no avail.
-Windows 7 64 sp1
-Adobe Creative Suite 4 & 5
-vray for rhino
-Windows up to date!
You might also check if your .NET version is higher than 4.0--this is a known issue which can cause ribbon problems.