The needle on my sewing machine literally just fell off. Not kidding. I'm holding an unbent and otherwise pristine needle. Oh brother SE-270D! I'm not sure how to put the needle back in. I think it just needs to be put back & tightened but I'm not sure how..

When you put the needle in the needle clamp holder the flat side of the needle goes toward the back of the the machine when you tighten the needle clamp screw make sure it's holding the needle,,make sure you can't pull the needle back out of the clamp,, if you can something is wrong with the needle clamp or the screw

Yep...just push the needle back up into the opening...note that the flat part of the top of the needle faces the back of the machine. Then tighten the screw. ALSO, there is a tiny hole on the needle bar, just above where the needle goes...you should see the tip top of the needle in that opening...if not, slighty loosen the screw (1/2 turn)...and push up until the top of the needle is showing...tighten the screw.