Are you using the original Wyse keyboard? Other than that I guess you won’t be able to access the BIOS.
To reflash the unit you could try Wyse USB Imaging (but I guess S30 is not supported) or Wyse Simple Image (to be found on this website).
Try booting the unit that way:
– unplug power
– plug in power again
– repeat pressing the “p” key while powering the unit on
– does it show some boot screen?