No expert. Having issues with 450d (1 Viewer)

I purchased a Canon 450d on ebay early this year. It has been great until I went on holiday a few weeks back and now I'm having nothing but problems. The camera works fine in autofocus but as soon as I change to av to take shots of birds it goes crazy. I rarely change from iso 100-400 always in f5-6 using a sigma fixed 400 lens. Before the holiday all was okay now its driving me crazy with focus issues ie nothing is in focus. As I state in the title I'm learning as I go so if anyone can help me I would really appreciate it. Many thanks.

Carl

I believe your problem has something to do with the fact that the Sig 400 f/5.6 is an old lens and isn't compatible with newer bodies unless they have been rechipped or the software has been updated. Just my $.02

I too suspect the lens, do you have access to another lens to try? If the AF problem is only with the Sigma then you know it's not your camera.
I note you said that this happens in AV mode, have you tried TV mode as a comparison? TV may be a good option as shutter speed is often more important than aperture for birding.

I will give it a go and many thanks to you both, as this was really getting me down.