That needle should have something to secure it to/with the diaphgragm.
I stole the below picture from the interwebs, but it illustrates generally what you should have, and the order they should be in. Think of the diaphragm as the bottom of this assembly. Everything else should drop in above it and then secure the cap on top of all that. If the needle just dropped, you are missing parts, or lost parts on disassembly.
The reason it won't run is that the needle is blocking the air/fuel flow. The reason it runs on the choke is because that part of the carb is a different circuit typically.
