Samsung Galaxy S :: Unable To See Downloaded / Installed Applications

I'm a newbie on smartfones and had spent a few days adding useful apps to my Galaxy S. File management and, file transfer applications to wireless link to my laptop followed by the transfer of all my work presentations (powerpoint) and lecture notes. All worked fine until today. Now, my device seems to have lost all the downloaded and installed apps (leaving gaps on the desktop). It has also discarded the image I had chosen for my screen background, and I can't see an icon to go to the Android store. Finally: on one of the screens I have the message "Problem loading widget". That might be connected to the problem that Android Market has completely disappeared. How do I get that back!! It's important. My transferred files are still sitting in folders in DCIM so that's good. but none of the special applications are visible. Can someone give an opinion on this. Has something taken place online with Samsung that has put my system back to its out-of-box state.