Double check that the wire diameter is compatible with the gun liner and feed rollers and matches the contact tip size.
.045 wire won't feed through an .035 gun liner or contact tip.
Also Double check that you didn't kink the gun liner. That will mess up being able to feed the wire through the liner.
Same with a 'dirty' gun liner. Blow it out, clean it, or replace it as needed.
Check that the end of the gun hose is properly seated all the way in the machine.
Used wire with a 'ball' on the end or a bend/kink in the end of the wire also won't usually feed through a gun liner. Snip the end of the wire back to where the wire is 'good' and not bent or deformed in any way.