well... if it is truly frozen then hold the power button for 10sec to force it off, then press it again normally to turn it back on.
Sounds to me (and this is a wild guess as you gave us no detail) like explorer.exe crashed. ctrl+alt+del, control panel, processes, rt click explorer.exe and select 'end task'. Wait a moment to see if it comes back on its own; If not then "file", "Run/New Task", explorer.exe, enter.