Note: This problem can be caused by ‘malware’ but try this:
As the Windows Orb (Start) is missing, open regedit via Task Manager by pressing Ctrl + Shift +Esc then on the Applications Tab click ‘New Task’ at the bottom, type regedit.exe and press Enter.
Navigate to registry key HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\Winlogon. In the right pane, make sure there is a REG_SZ named Shell and it has a value of "explorer.exe" (no quotation marks). If it’s not there, create it by clicking Edit > New > String Value then right-click it and key in explorer.exe then restart the computer.
To be prudent, create a system restore point (rstrui.exe in Task Manager) before making any changes to the registry.