If you were using a HiFiBerryAMP (the amplifier card from HiFiBerry) please update to piCorePlayer 1.18b, otherwise your card was not recognised after a reboot.
It has been fixed now.
The pCP Team
Paul, Greg, Ralphy and Steen
Older versions of piCorePlayer may not be compatible with the newer versions of the Raspberry Pi. You need to review the Release notes to determine when the support for your particular Raspberry Pi was added. It is safer to use the latest version of piCorePlayer as it will be compatible with the latest version of the Raspberry Pi.