focus issues

[pepe]

I was taking some pictures of my friends (they wanted some of their baby & 3 old daughter). I normally have great, sharp pictures. THey started out like this, and then, after some water shots (fountain) they got really blurry, I couldn't find a focal point. This was on auto focus.
I went to my menu and tried single point auto, and that didn't help. I missed some really great shots, because I couldn't get it to focus.

Any ideas why this happened, and how to fix it?
Nikon d80 Kit lens.
Thanks

[work2well]

Hi, can you post some of them, so we can try to see what went wrong? Along with exif info.

Thanks.