Do you get any errors in your System or Appliocation log after trying to shutdown? Generally a failed shutdown will place a USER32 Event ID 1073 in you system event log.
The attempt to power of failed.....
However this does not generally help to tell you what is happening - it more so lets you know the actual shutdown / log off attempt you tried was recognised by the system instead of not executing / being ignored.
Another command you could try and see if it helps is:
Click Start - Run
type "shutdown -s -f -t 05" wihtout the ""
Click OK
A box should appear telling you that it is going to shutdown in 5 seconds. -s = shutdown -f = force all programs to close -t = time. You could use -r for restart instead of -s.
If a screen flashes and disappears you have either typed the line incorrectly or you have issues because shutdown.exe is not being found.
To troubleshoot should this happen:
Click Start - Run
type "cmd" wihtout the ""
Click OK
type "shutdown -s -f -t 05" wihtout the ""
press key
It will then tell you whetehr shutdown was a recognised command or not. If it is not a recognised command eitehr shutdown.exe is missing, not in the path allowing exectution from anywheer on the PC or something is blocking its execution.
Anyway, give that a try and see if it works.