Boiler stopped, water pouring out the overflow

Hi all, new here and looking for help as you might have guessed !

Here's the problem. Just before Christmas I noticed my oil boiler was making an unusual whining sound, plus it was no longer heating up the hot-water cylinder, and putting on the immersion only heated up the top five inches or so of the cylinder.
On Christmas eve just as I was leaving, it was about -5c and I turned on the boiler in the morning. It worked for about 20 minutes, then shut down and wouldn't restart but I hadn't time to investigate as I had a plane to catch.

I've just arrived back after being away since then and the boiler refuses to start. There's plenty of oil and I don't think it's frozen. I've pressed the reset several times but nothing happens, no sorta roar as usual as it gets going, and no sorta sucky/clicky sound like it's out of oil, however water absolutely pours out the overflow under the gutter.

I had a look in the attic and water was flowing out of a u-shaped pipe into the main tank at a rate of knots, which this was then overflowing.

Any ideas what's going on ? Could it be something like the coil in the cylinder has split, or the pump on the boiler has gone ? I'm not sure it's the pump since there's this great flow of water and yet the pump and boiler don't seem to be doing anything.

Help much appreciated because I'm freezing and had to turn off the water because I did not like that it's about two inches away from flooding my attic and house even though it was venting properly.

SOme things about boilers are common the world around, but not all, so trying to understand your configuration in Ireland may take a bit of education on both sides.

Boilers have multiple safety interlocks. IF you have a major leak, it may not be above the minimum pressure, and won't turn on. So, you need to figure out why it is leaking. once you figure that out, then you can troubleshoot why it won't fire up. If you are competent with a meter, you could verify the state of the interlocks and hopefully, there is a pressure gauge you can check to see what the pressure really is. Other than that, can't help more without more info.

Thanks for the reply Jadnashua. It's not a pressurised system, I think it's what's called open-vented, and has an indirect hot-water cylinder. What's confusing me is where the water is coming from if the boiler isn't turning on, I can't trace where the pipe is coming from, but I suspect it's off the top of the hot water cylinder.

Sounds like you have an EXPANSION TANK in your attic space. Check to see if you have an automatic feed. (you shoudn't) but if you do, it may have failed and is pushing water into your system . The non-pressurized systems that I have seen have to be filled manually(open and shut off valves by hand).

As far as the boiler firing, the mention of a pump throws me off.. Is it possible you have a flow switch. If the pump is not working, the flow switch my not be activating the boiler to fire.

As previously mentioned , we need to know more about surrounding controls ,( if any) on your system. More info, or a picture of any related components of your system would be helpful