I have had the same problem and occasionally it happens again but it’s quite simple to rectify. You need to find your CATSettings folder. Mine is in ‘Winnt40/Profiles/your user name/application Data/DassaultSystems/’. Within this file as the name suggests are all the files .CATSettings. I create a ZIP file of all these files (just in case) then delete all the files. I then start Catia and ‘BINGO’ all is fixed and back to normal. If you have customized any toolbars you will have lost these. But at least all the buttons have re appeared. I have made a zip file of all my CATSettings when everything is working perfectly and when the problem re occurs I just delete the troublesome settings and unzip my settings and everything works fine.