The machine won't shutdown

I have used the latest stable version of OMV and all packages are up-to-date.The kernel is backports(3.16).Recently i found this problem and the machine worked well before.It wont shutdown when i choose the button on webUI.OMV service has stopped but debian still works.Even i use the command "shutdown -h now" in ssh,it wont power off as well.The only way to turn it off is holding the power button for several seconds to force it halted.However,this way is not healthy for PC.