Reboot not work on Dell OptiPlex 750 desktop at Ubuntu Natty distro

[Sharl.Jimh.Tsin]

hi,all:
     i am using Ubuntu 11.04 Natty on my office's desktop which model is 
Dell OptiPlex 750 serials.recently i found that the "reboot" function of 
this OS never worked,it will make the OS hanged and with no response of 
the keyboard or mouse. only the Power button can kill the program.
     So,i just want to know that Is it a BUG of Power Manager or 
anything else? does someone also face this problem same with me?
     want a reply,thanks~

-- 
Best regards,
Sharl.Jimh.Tsin (From China *Obviously Taiwan INCLUDED*)

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