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.