Make sure that your machine is threaded correctly and that the bobbin is inserted properly too. Also put a new needle in the machine.....cross your fingers. I hope this works!

Rethread your machine making sure that the thread is in the tension properly, also do the same on the bobbin

Change your needle and give your machine a good oiling. Also rethread uppper and lower cottons. Make sure that you have the proper tension on your machine. You can check that on your help bar. I hope you resolve this soon.

Make sure there is no thread shredded somewhere in the machine. I panicked when this happened to me and took my machine off to the sewing store. All he did was unscrew the front where the thread runs through and there was a small piece of shredded thread caught. He did tell me it was Sulky Rayon and you shouldn't use that but I have talked to lots of people who say they have used Sulky and/or rayon for years. Easy fix and now I can do it, too.

If you get a birds nest on the bottom - it is your top thread. If you have changed your needle, and the bobbin is in correctly take the top thread out and start all over. Many times threads will skip the arm and slip off causing this issue. Hope you got it fixed.

Make sure when you thread the machine that the pressure foot is up then the thread will sit in the tensions properly.

Make sure you are threading all the places in the thread path you are suppose to such as the take up hook.

What fabric and stabilizer are you using and needle size?

Check to see if you have any thread that caught in behind the take up lever inside the machine.

Check to see if there is any thread pieces in the bobbin tension on the bobbin case. You can floss the bobbin tension back and forth with regular thread to clean any bits or dust out of it.
Make sure to clean the bobbin area often.

EastWitch2

This could be a teeny tiny piece of thread stuck in your tension discs. When you change colors snip the thread at the spool and draw it down and out thru the needle. A lot of people just pull the spool off the machine and the thread with it. It's like flossing backwards when you do that. You can try using a piece of UNWAXED dental floss and running it thru the threading system. Miniscule snags of thread fibers can get trapped between the tension discs and cause problems. Rayon is a weaker thread and more likely to shed tiny pieces of fluff.

In the "old days" machines were not computerized and machined so tightly and they were not as likely to be fussy about how things were done. Remember when your mother or grandmother would have more than one color of thread on the bobbin? You could change your thread then by pulling it backwards but that machine couldn't do 1/4 of the things they do now.

You may want to make sure your needle has not gone thru the top of what passes for a bobbin case these days. It will cause a tiny rough place and thst could cause a snag also. On my Brother machines this usually is hiding in the strip of felt like material that guides the thread. If you hold it up to the light and look - it should be solid, no pinholes. Not at my machine right now so can't describe it any better, sorry. Dull needles can deflect and do this without making a big deal at the time so you may not realize it. And how do I know this .....???

Your problem will always show reversed - bobbin nest - top thread problem, a top thread problem will be related to the bobbin.

Hoping you are all better now. Don't forget to pat your machine and thank it after you are finished sewing - I swear they can get PMS too.

When this sort of thing happens to me, I start all over. Reinsert needle, rethread, take out bobbin and insert it again.

Look at your bobbin case, You probably have a mess around it, too. Take it all out before you begin again, with a new needle.

I make sure my tension is taken up by moving the thread back and forth a couple of times before the needle.

This happens to me when I put my bobbin in up-side-down (yes, I STILL accidentally do this lol) .. hope your solution is as simple as this and you can get a-stitchin' again :) xXx

The only thing I can add to what has already been suggested is: Are you using sticky stabilizer? If so, a bit of glue on the needle will keep breaking the thread or create the thread nests underneath the fabric. I keep a Q-tip and bottle of alcohol on my sewing table to clean the needle and dry it well before rethreading.
Angel

Do you have the right size bobbin in? I now make sure I mark and separate the A's from the L's. Just that little bit makes a big difference.