After doing all checks as suggested by previous posters(don`t forget to see if any core plugs have been pushed out too) .......if you can get an electric cable to your engine you could try a small electric heater under the bonnet....but don`t let heater get wet,make sure you keep an eye on it and switch on/off at mains,not at heater,use circuit breaker and don`t touch car or heater whilst it`s thawing.Then use warm(not hot) water to top up and see what leaks appear.If none,then drain and flush complete system incl reverse flushing rad&heater matrix.Use at least 30% antifreeze in mix when satisfied and having already run with no leaks with just water,or else you might just be wasting your money.

If you have been running it a long time without antifreeze,chances are the waterways are badly corroded,rad /heater matrix blocked with crud hence importance to flush engine thoroughly.Sounds like it was only a matter of time before the head gasket let go anyway.Would be very surprised if you`ve got away with it as it was well below freezing for a long continued period but good luck