Most cars run on 12 V DC. A 250 V fuse wouldn't work properly in such a system. You can check the A/C fuse under the hood and see its rating - I'd assume it's 12 V, maybe 20 A.

I think your air conditioner problem isn't related to that fuse.

In what way does your A/C not work? Does the fan not blow, or does the A/C compressor not engage, or does it not blow cold air out the vents? It could be lots of things (a switch problem, being low on Freon due to a leak, temperature sensors in various places not working properly, etc.).

Hope this helps a bit.

Cheers,
Scott.