Maya 11 Missing Buttons

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Those buttons aren't that important... (although I have to admit I don't know how to make them reappear either ^^) They're just shortcuts to different viewport layouts. You can also access all viewport layouts by clicking on "panel" in the little bar right above your viewport.
Looking at your screenshots, the problem might simply be your screen resolution - Maya doesn't have enough space to display all buttons so it cuts some away...?

EDIT: Can somebody tell me why i somehow sometimes cannot select any model i've made. cube or sphere, when i click it, nothing happens

a) They could be on a locked layer. Check the display layer window under your channel box. If a layer there has a little "R" or "T" next to it it is locked as a reference. Click on the letter until it disappears to unlock the layer.

b) You might be in sub objectmode. F8 toggles between object mode (selects objects) and subobject mode (selects faces, vertices and the like).

c) Maya can make certain types of objects unselectable to help with cluttered scenes. You may have accidentally deactivated the selection of polygon objects there. Check the little buttons above your shelf:
http://s8.postimage.org/ls0ia56c3/mayabuttons.jpg

d) If nothing else helps, see if you can select the object in the outliner instead. (windows> outliner)

Last but not least: Whenever Maya is acting strangely, it often helps to delete the userprefs - file. It can usually be found under User/Yourname/Documents/Maya/2011-x64/prefs.
That file gets easily corrupted and can cause all sorts of weird behaviour if it is. Deleting or moving it will cause Maya to create a new one, basically resetting the program to its pristine state.