RichC is right. Even a freshly connected keg should dispense, carbonated or not.
If you pull the pressure release on the keg, does it hiss? If not, your gas diptube, poppet, or quick disconnect is the problem. Take them apart and figure out which it is.
If it hisses, it's your liquid diptube, poppet, or quick disconnect. Take them apart and figure out which it is.
To take any QD apart, remove it from the keg and unscrew the plastic screw on top. Reassemble after cleaning/sanitizing everything.
To take any poppet/diptube assembly apart, disconnect the gas and liquid QDs, pull the pressure release and keep it open to allow the keg to vent, and leave it open so liquid doesn't come out of the tubes. Remove the posts, inspect poppets. Remove the diptubes, inspect them. Clear out any blockage, then sanitize everything and reassemble. Put gas back on the keg, and vent it a few times to purge any O2 that got i.