Plumbing – Repressurising Logic Combi 24 boiler

Step zero for the future (you've figured this out by now, but let's state it loud and clear) - extreme cold is when you don't want a setback at all - I shifted my thermostat from its usual automatic to "hold temp" last night (same cold front) for that reason. You can also have a circuit installed that runs the circulator every so often regardless of call for heat as another way to prevent this.

There is normally a valve to fill the system, it will be external to the boiler, and it may well not be automatic (or there may be an automatic and a manual in series, with the manual normally shut.) There is usually but not always an "anti-backflow preventer" as part of the fill system. There may also be something that looks like a pressure regulator, with a toggle on it that can be manually manipulated.

In front is the overflow pipe from the relief valve, behind is the "pressure-regulator-looking" fill valve (the part that's manipulatable is just off the picture, sorry - a toggle you can lift to force some flow) and below that is a manual valve that has to be open to fill the boiler. The entry location is typical (right at the bottom of the boiler) but that's not a given.

Circulator pumps don't do well with air in the system. You will need to replace the water that's leaked out AND remove air from the system at bleeder valves (typically found on elbows in high parts of the system)

There should be a temperature/pressure gauge on the boiler. You typically want 20-30PSI showing on that.

You could have a frozen pipe (actually, it would take two in this case to completely isolate the upstairs bleeders.) You might not. Don't know the layout of your pipes...

Circulation pump does not need to be running for bleeding. System pressure alone should suffice for bleeding. "Hissing" when you bled before sounds like trapped air. When you start to get water mixed in, close the bleeder, wait a few minutes and re-open. You should eventually get just water, no hiss, no bubbles.

I'm going to suspect that you have low pressure in the system, so air is not being forced out the top bleeders by water pressure in the system. You should have a pressure gauge (often on the same face as a temperature gauge) on the boiler - what does that read? Normally it should be 20-30 PSI. You may need to add water.

If you can't sort it out yourself, call for professional help sooner rather than later, as letting the system freeze can result in considerable expense.