Has it ever lost pressure before? Have a good look around the system (radiators etc) for any obvious signs of a leak/weeping. May be worth trying to pressurise it again and see if it holds. Most boilers are easy to top up pressure.
There is a potential reason for total pressure loss. If the expansion chamber has failed and the pressure relief has operated. The pressure relief valve often doesn't seal again properly after that.
Get home from a trip to the house at 12 degrees, no water pressure in boiler and a boiler that won’t light.
Any one got any hot ideas while I have a meltdown and put off the usual phone around on a Monday to discover no plumbers are available…?
Baxi in BR1.