How can I investigate the cause of total hang? Is there some information that I should pay attention to in order to get some hint?

[孙世龙 sunshilong]

Thanks to all of you.

To Colin Law:
>Either it is crashed or as others suggested suspended, in which case
>there should be an indication of that.
>If crashed then look in the log as I suggested.
I think it's a good idea.
I have checked the /var/log/kern* carefully, but I have not found any useful
information.

To Oleg Cherkasov & Mike Marchywka:
>Very good suggestion indeed to unplug all USB devices if possible.  I
>had few times a problem with an external USB disk.  It started to fail
>and endless re-syncing almost hang my system so it way nearly unresponsive.
There is no usb devices except keyboard and mouse. I'd try unplug them.
>BTW does "Magic SysRq" work as expected after the system hangs?
For details, see below. Thank you.

To Liam Proven & Mike Marchywka:
>Try the magic REISUB key combination.
>https://en.wikipedia.org/wiki/Magic_SysRq_key
>Update your motherboard firmware.
>Is it a desktop or a laptop?
Sometimes the magic SysRq key works, sometimes not.
And only limmited functions could be provided by SysRq key,e.g.:
see the memory, reboot the system.I don't see any useful function,
though I have enabled the all possible functions.
>Update your motherboard firmware.
I can't do it indeed.
>Is it a desktop or a laptop?
IPC(Industrial Personal Computer)

Colin Law <clanlaw at gmail.com> 于2020年7月2日周四 下午10:55写道：
>
> On Thu, 2 Jul 2020 at 14:28, 孙世龙 sunshilong <sunshilong369 at gmail.com> wrote:
> >
> > Hi，
> > Thanks to both of you.
> >
> > >Does the Caps Lock light still go on and off when you hit the key?  If
> > >not then the machine has probably crashed completely.
> > >On reboot look in /var/log/syslog to see what it says.
> >
> >
> > >Try to switch into console mode with Alt+F3 or Alt+F4, login and check
> > >dmesg messages if there are any.
> >
> > No response to any key(e.g. Caps Lock, Alt+F3, etc).
> > Any more methods that I should try.
>
> Either it is crashed or as others suggested suspended, in which case
> there should be an indication of that.
> If crashed then look in the log as I suggested
>
> Colin