Does it appear to attempt focusing in the preview (aka "hunting") but never achieve a focus lock? Have you tried manual mode and manually adjusting the focus? If so, do you see any difference or can you get a sharp image? What if you view things at different distances, can you get anything more or less in focus?
A stuck focus motor can be one possibility. It could also be a scratched lens, or even a lighting/environmental issue. Registering so you can give us more detail will help us narrow things down. You'll also be able to upload a sample photo so we can see what you see and understand the problem better.