Hi Anas,
All the material and geometry links in the Sandbox editor are relative ... this means that when you place a model or assign a texture it saves the path to that asset) rather than literally placing it into the environment.
So if the pathways aren't the same (or the texture or model is not on the new computer) you'll get the replace me texture or a yellow ball (as a placeholder) instead of the model.
One way to fix this is to use the material editor to reallocate a material or in the rollup bar of the geometry in question (represented by the yellow placeholder ball) you can scroll down and click on the model link and then navigate to your model (which should be in your levels folder.
For your terrain problem, the simplest solution would be to create a new terrain layer and repaint it.
Cheers