Parts disappearing from drawing but show up in the Preview in Windows

So it's been a reoccurring problem for me I have multiple layers of different parts of a house i've been creating for college. Firstly it was my ground floor walls now it's a worktop I created from a few 3D Boxes which I Shelled and Union-ed. I created it for a kitchen counter / worktop. I have a back up file of the file saved in two separate locations and the part has in both of them and from any older versions of the file it wont let me Copy / Paste them in using windows copy / paste method but the copy works if I try to put it in to a completely new file. Don't know if anyone has had any problems like this before but i'll attach 3 images one showing the preview window showing the counter, one showing the file open and the counter not being there and an oldish version of what should have been there. Any help with this problem would be greatly appreciated as the last time when my wall vanished I had to create a whole new file and copy / paste everything in to it to be able to copy the walls from an older file. Cad2014- Part Vanished - http://i60.tinypic.com/elcz1i.png Win7 View - Preview shows it as being there - http://i57.tinypic.com/fa332q.png Cad2014 - Old Version to show what should be there - http://i62.tinypic.com/35n5ulv.pngImage may be NSFW.
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