This week both PiCorePlayer were silence.
(Off course in the morning when my girlfriend tried to turn on the radio and she was unhappy)
I was able to access the PiCorePlayer URL.
It looked like "Squeezelite was running"
Did a restart of Squeezelite and got the message it was not running
A reboot was required to solve the problem.

What could have been the root cause of this problem?
How can I prevent this?

PI3B+ LMS + PiCorePlayer (wired/HDMI)
Zero: PiCorePlayer + HiFiberry DAC (wifi)
piCorePlayer v8.1.0 | www v00017 | linux 5.10.77-pcpCore (32) | piCore v13.1 | Squeezelite v1.9.9-1392-pCP
Logitech Media Server Version: 8.2.0 - 1627922070 @ Tue Aug 3 11:43:18 CEST 2021
For controls I use OpenHABian