I recently had a problem with my cornelius keg. A few days after putting my IPA in my keg, it got clogged. I looked on the internet but found nothing that could help me so I decided to to post a quick solution if it happens to someone.
Actually, it turned out that some hops had clogged the sucking pipe in my keg. What you have to do is to unscrew the fitting on the keg. It can be done with a 7/8" socket. This will remove the fitting as well as the check valve. Make sure nothing clogged any of it. After that, you can remove the pipe simply by pulling it. The clog is probably in the tube, then just blow in it to remove the clog. If it doesn't work, you can try with a air blower gun, that should do it.
If by any chance the clog isn't it the pipe nor the fitting, it might be the tap, but I would bet that the clog is in the pipe!
** Btw, this solution implies that you have to remove the beer from the keg for a few minutes...
Hope that helped someone!