Why has my autofocus gone mental?

[Ronnie the Raincoat]

I have searched google, now I turn to whores.

I have a Canon 450D and for some reason, the red AF points have disappeared.  It's not a low battery or anything like that, and it's not on manual focus.  It's also not the lens, as I've tried two different ones.  It occasionally flashes up, but much dimmer, like half the brightness, and doesn't stay on.  It also isn't focusing well at all and most of my photos are coming out badly focused.

Any ideas?

[Cerys]

The cat has snaffled them.

[Blumf]

Some guesses:

Is this in all shooting modes?
Are you 100%, no go check again, sure you haven't gone into select focal point mode?
Check the AF switch on the lens(es)

Personally I've had no end of problem with Canon's autofocus. Never works properly, always goes off and cycles through the whole range of focus before landing on something that clearly isn't relevant to the shot.

[Zero Gravitas]

You haven't got the focus set to use just one of the of the edge points have you?

[sproggy]

Or nearest subject, which BTW is fucking rubbish.

[Ronnie the Raincoat]

It's in all shooting modes, on all lenses, it's switched to AF, and it's on all AF points.  But other AF points other than the bloody central one seem to not be working.  When I select a different one, it doesn't work at all.

[Ignatius_S]

Have you checked the mirror? I think if that gets dirty, it can cause similar problems.

Also, although you've tried two lenses, it might be worth checking the conection areas between them and the camera - it's a bit of a longshot, but I think dirty connections can cause AF problems.