Help.... Serger won?t make a chain.

It's very probable that you have put the timing out when it jammed. It often happens because your blade is dull and the fabric being cut catches into the stitching. You could try retreading and changing the needles but your best bet is to go to a service person

Since you have already done the easiest things (new needle and re-threading), I suspect you bent or pulled a looper out of position. Your best bet is a qualified service person.

At this point, I would make one more try before taking it to the shop for possible repair from the jam.

1. Take all the thread off the machine, cones off the spindles, the whole bit. Clean the machine as directed in your manual. Put in new needle(s) making sure it or they are in right way around and fully up in the needle clamp. Yes, I know you just put in a new one. Humor me, and put in different new one(s), making sure they are the right needle system for your machine. Some sergers can take standard sewing machine needles, others cannot.
2. Open the manual to the section on threading and rethread carefully, reading each step out loud before you do it. (reading a step aloud seems to get the brain and fingers in synch better than just reading'to yourself.) Make sure you thread in the order specfied, because it gets the internal thread crossings made correctly. Make sure if your machine needs to have the presser foot up when threading, that you do that.
3. Pull all the threads under and behind the (raised) presser foot and hold them together. I don't know where I picked the habit up, but I twist the threads together a little, like a rope. Drop the presser foot, and holding the threads behind the presser foot, step on the gas while you lightly pull on the threads. The chain should start forming, and at that point, you can drop the thread ends and just sew.

If a careful rethreading does not fix the "doesn't chain problem), double check the needles are the right system for the machine, and inserted correctly and fully. If your machine takes round-shank needles (as opposed to the standard sewing machine needle type with a flat spot on the shank) make sure the needle is correctly oriented. Even the round shank needles have a front and a back to them; they are not symmetric. Also rethread one more time from scratch.

99% of the "not chaining" problems are incorrect threading, incorrect threading order, incorrect needle or damaged needle. The other 1% are generally things you need the repair shop for... a bent looper, an out of time looper, a missing or bent stitch finger...