Sometimes when I'm working in Illustrator I'll accidentally hit a button that changes my window. Tools across the top disappear and sometimes other tools I use. It takes me a while to figure out how to get everything back, but I usally do.
Other times, like a short time ago, my tool bar reverted to a minimal amount of tools. When using the color palette, it switches from CMYK, my document setting, to RGB. I have to switch it back to cmyk over and over withing the same document I'm working in. Little things like this continue happening.
Dear Adobe, can you please slow down the updates and new features? I know I have the option, at least I think I do, of downloading updates, but I don't like being asked to, over and over. I just want to be left alone and work. Years ago I never had any of the problems/issues I experience today with Illustrator, before the subscription method. I only updated to newer versions at most every 3-5 years. I don't need the latest features every other day to be shoved into my face. I say that in the nicest way of course...lol.
I don't always think it's my lack of understanding setting up Illustrator the way I like using it, as in color mode in the document settings. Too many weird things happen, like defaulting to original Illustrator settings, nearly every time I open existing documents, or create new ones. Have other users mentioned this before?
Draw and design
as for the Control Bar right below the menu bar disappearing you probably hit the Tab key. When you do everything disappears, tool box, control box, panels, etc, except the Menu bar. If that's the case, hit Tab again to bring everything back.
you may also be accidentally hitting the F key to go into the different Views. Hit it again to cycle through Desktop, MultiWindow and Full Screen Mode.