Try to open the task manager , by pressing Ctrl+Shift+Esc key altogether. Open , New task and enter "explorer.exe" in RUn.
maybe the Explorer is not starting automatically.
There could be many other reasons too. But , you need to post some more details.
forget to mention - before doing any repair/reinstall just check your pc with some latest antivirus CD just to be sure that some kind of virus is not causing this.