The needle shaft on my brother se 400 will not move up or down

Have you tried a brand new needle? Is the needle installed facing the correct direction? If this doesn't help, then you may need to have your machine serviced as it could be out of alignment.

The needle threader is a very delicate item on the machine. What is your question? As always, please have a trained technician make the repair for you, they have access to any parts you may need and they can do it safely too!
Cheers,
Embroidery Services
San Diego, CA

TRY TO REMOVE YOUR BOBBIN CARRIER AND NEEDLE AND SEE IF THE MACHINE WILL MOVE SMOOTHLY WITHOUT THEM. IF IT IS RUNNING SMOOTHLY YOU KNOW YOUR PROBLEM IS THE BOBBIN CARRIER OR NEEDLE. YOU MUST HAVE THE RIGHT SIZE BOBBIN. ONCE YOU HAVE THE RIGHT BOBBIN YOU CAN PLACE ONE DROP OF OIL ON THE RACE (LITTLE TRACK UNDER BOBBIN CARRIER) AND PUT THE CARRIER IN. WITH YOUR HAND TURN THE HAND WHEEL TO SEE IF IT IS INSERTED PROPERLY SO IT ROTATES SMOOTHLY. THEY'RE IS A PLACE THAT KEEPS THE CARRIER FROM MOVING. YOU MUST BE CAREFUL TO LEAVE IT LOOSE SO THREAD CAN GO UNDER IT. IF IT IS STILL MOVING SMOOTH WITH YOUR HAND YOU CAN TRY THE PEDAL. IT SHOULD WORK. NOW, REPLACE THE BOBBIN MAKING SURE IT IS ROTATING THE RIGHT DIRECTION. I CAN'T STRESS ENOUGH TO MAKE SURE THE BOBBIN IS THE RIGHT SIZE. IF THIS IS OK THEN REPLACE YOUR NEEDLE. THE NEXT STEP IS TO GO TO YOUR REPAIRMAN IF IT IS STILL A PROBLEM.

Rather suspect the bar has bent, remove it and check for true

try changing needles
also possible that the machine is out of time

The needle breaks
1. The needle is incorrectly inserted
2. The needle clamp screw is loose
3. The is bent or blunt
4. The needle is too fine for the fabric being used

Return the bobbin winding shaft on the top right of the machine to the Left Position. It may be stuck in the middle.
I had this same problem. After an hour or two of fiddling, this fixed my problem.

needle might not be fully pushed up in shaft, or wrong size bobbin is used

Make sure the needle is all the way up in the shaft/bar. It can work loose and slip down. Make sure you're installing the needle correctly.

see my notes on your premium solution. I'm around the 4th of 5th down the list.