If you're on a terminal connected to the serial console port, try pressing Control-B (after you've followed Bill's advice). The GSP should then answer to you even if the OS is hung.
If you can get to the GSP> prompt, the command HE LI(for HElp LIst) will list all the available commands.
The VFP command will display the state of the system LEDs. It will also ask if you want to activate "live mode VFP". If you answer Y, it should start printing four-character hex codes on the display until you exit the display. If one of the hex codes is something like FxxF (where x can be any number), the OS is still alive and just cannot respond on the console for some reason. Other codes might indicate the system is crashing/has crashed.
If the VFP indicates the system is down, return to the GSP> prompt and use SL to view the hardware error log. Note that the hardware error log uses UTC timestamps.
If the operating system is still running (you get FxxF codes in VFP Live mode), but you cannot wait to see if it eventually recovers on its own, you can use the TC command in the GSP> prompt to TOC (crash & reboot) the system. It will cause the system to create a crash dump, which can be analyzed to see what caused the system to become unresponsive.
Initially, the crash dump will be written to the swap partition by default; when the system reboots, the crash dump will be compressed and moved to /var/adm/crash before the swap is activated again.
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