Quite new to this - sorry

While using Rune with Airplay the system froze - the two same notes started playing in a loop. We shut down Airplay, but it kept on playing the same loop. The webpage (192.168...) went unaccessible. Impossible to connect via SSH (connection timed out). No response either when we attached a screen to the Raspberry Pi. At this point I'm leaning towards unplugging the whole thing but I don't want to damage the system, however I couldn't find a clear explanation on how to safely reboot it when the system has become this much unresponsive.
Thanks for your kind help!