Hello and welcome.
In addition to honeston's question/comment:
-Do you see the red and green buttons, but not the yellow?
-Does the top menu bar appear if you move your mouse cursor to the top of the screen?
It sounds to me like you may have applications running in full screen mode. If so, press the Esc key and the app will exit full screen mode and you should see the menu bar and window control buttons again. If Esc doesn't work, try moving the mouse to the top so the menu bar appears, then click the green button in the upper left of the window. Also, when in full screen mode, the yellow button usually turns grey, but will turn yellow again after exiting full screen mode.
Here is a little more on full screen mode:
OS X Yosemite: Take apps full screen
Let us know if the issue persists.
C