@ ctbram: thanks a lot!

thank all of you for your patience and kindness, now i have the solution...

somehow i have to select the mesh in the outliner and then do something with it (i.e. scale, move etc.) in order to make it reappear...it's like the object is in sleepmode until i grab it^^
i have no idea whether this is normal or not, i assume it has to be some kind of bug...
but i don't care, as long as i am able to go on with my work...