Main combi 24HE FLAME ERROR NO HEATING OR HOT WATER, PRESSURE IS AT 0, HELP?

hi can someone please help, our boiler isn't working, there is a flame error and the pressure gauge is at 0, with this freezing cold weather it couldn't have come at a worse time. Can someone please help me fix this problem as i have guests coming tomorrow too and its completely freezing! Please also let me know while i'm doing the repair should the boiler be switched on or off, and will the pressure gauge work even if the boiler is switched off. Thank you, a speedy reply will be really appreciated. Thank you