Default [Q] stuck in fastboot, hold down power does not help

Hi,

I got in fastboot by accident while rebooting for a switch back from ART to Dalvik on OmniROM.
But it appears to be completely stuck. It shows ***fastboot... and notihing happens.
Hard reset by holding down power does nothing (tried for 10secs, 20secs, 60secs).
adb does not detect the device.
My last resort: wait unitil the battery is empty.
Does anyone have a better solution?

Many thanks,

Bastiaan