I cannot see my codes anymore

You may have deselected specific coding stripes. Click on the gear icon or right-click the grey area where the coding stripes normally appear to open a window in which you can select which codings you would like displayed there. If you want to display all of them, make sure, all the boxes are checked. In the preferences of the Document Browser window you can choose whether your coding stripes filters should persist when starting MAXQDA or whether you want to reset your coding stripes filters when starting MAXQDA.