By "out of the top" do you mean between the regulator and the bottle, or out of the top of the regulator?
If it's coming between the regulator and the bottle, then it's most likely started to unscrew itself.
If it's coming out of the top of the regulator, then have a look at the pin in the middle , if it's not sticking up completely, take something with a flat base which is small enough to fit in the hole (clothes pin works, end of a pen... something like that), and push the pin down (MAKE SURE THE TANK IS NOT FACING TOWARDS YOU OR SOMEONE ELSE) gently, if no air comes out, press it in a little more, then let go, and the pin *should* pop right back out to where it *should* be, and you *should* be able to fill your tank again.