Nikon camera (d5100) blurry photos on automatic setting?

I've had the d5000 for a few years and just bought the d5100 on a black Friday sale. I ordered a lens filter thing right after I got it, and it arrived like a week later. The first week, the camera took great pictures. As soon as I put the filter on, the pictures were all blurry on automatic setting. So I took the filter off and they are still blurry. I'm sure I didn't touch or scratch the lens while putting the filter on. I even cleaned my lens, and it's still blurry. It's the lens it comes with- the Nikkor 18-55.It's not the best lens out there, but it should still work, especially on automatic settings. Ideas on how I can fix this? Trying to take Christmas card photos of the kids and everything is garbage!

Dirty lenses don't cause blur unless you've been mud wrestling. Check that your lens is set to auto focus. There's a switch on the side of the lens. There's also a menu setting to set you camera to manual focus. See page 45 in your manual.

It might be the problem with the filter, what filter did you buy? Polarizer and and will cause light loss, and therefore slower shutter speed, it might be the reason why the pictures are blurry, if its a clear filter, check your autofocus switch and make sure you are on P or iAuto