Why isn't the A/C in my 13 year old Citroen C1 not working anymore?

you need to get it regassed 

The clutch for the A/C failed or it ran out of Freon.  Go see your mechanic for find out the problem and cost.

If it needs more than regassing, it’s going to cost more than the car’s worth. 

On the bright side, the engine is scarcely up to running a/c, so the car will go better without it, although that big windscreen does make it like a mobile greenhouse.

Probably because either something is no longer working and/or the Freon leaked out.

Because it fixed itself?

Your question contains a double negative (isn't/not) and says the opposite of what you're trying to convey.

Yes. Halfords Autocentre will do the job. Not the Halfords store. Google to find your local Autocentre. 

Check if the compressor clutch is engaging.  Check the charge.  Check for power and ground at the clutch when the AC is ON.  If you do not know how, see a mechanic.

get it professionally checked for a gas leak

It might just be out of ice.