Sounds like your lower knife may have become disengaged. Your manual should tell you how to engage it. Or the knife may have become dull or misaligned, which can happen if you've gone over a pin. If it's dull, it needs to be replaced. These are the only things I can think of.

The knife is fairly new, the only thing I did was replace the needle last week, and it hasn't worked the same since..when I do any serging, it looks like a rolled hem, and that's not what I want..would changing the needle have anything to do with it?

If you had said that first, then I would have said, yes, the needle could have everything to do with it. Threading, needles and tension are the main causes of any problem. I bought some Schmetz needles in bulk, cheap, once, and I don't know if they're slightly defective or if my serger just doesn't like them, but I can't serge anything if I use them. But when I use them from a different package, everything's fine again. Also, when I have to change threads, even tho I know how to thread it, and it all seems OK, I still have to rethread several times till it works right again. I think serger's are just finicky. So, just keep trying a different needle till it works again.

I have a Singer 14U234 which might be similar to yours. It's about 15 years old. Over the years from time to time I've had a similar problem after serging something very heavy. I found that the lower knife would be out of place so I would loosen the screw and straighten the knife and retighten the screw. Also did you raise the upper knife to change the needle and forget to lower it again? On my machine I have to use "serger" needles, not just ordinary machine needles. Does yours require special needles? A lot of the newer machines don't require "serger" needles. Just a few ideas.

You don't have the knife engaged. When you push the knob in and turn down, you will hear a click. Also note, as written in your instruction manual (you did check the manual?) that the needle has to be at its highest point for the knife to be adjusted, engaged and the all-good click. I was doing the same thing as you, checked the manual, and read that small detail I missed when I originally read the manual several times. Serger life has been good ever since.

A needle in backwards makes for problems too. I've done that more than once!