Joe's answer is great and I would rack to a new keg if all else fails. There's something simple to try first:
bleed off almost all the CO2 in the keg then blow out any hop material in the beer dip tube by connecting the CO2 tank to the beer out line. (You can put the CO2 connector to the beer out post on corny/firestone kegs, don't know about pin lock kegs.) Turn the pressure up until you hear the tube clear.
Use gelatin finings (if you're not adverse to that) to clear the beer and keep the hops at the bottom of the keg. It's important not to move the keg after the gelatin has settled, so put the keg in it's final serving place after adding the gelatin. Leave for 5 days.
Bleed off most of the CO2 again, and pour a half pint or two. Use minimal dispensing pressure so not to pick up any loose hop particles. With successive pints you can increase the CO2 pressure. You should notice each successive glass becomes clearer.
This isn't 100% guaranteed, but pretty simple, especially if you don't have a spare keg to rack into. Even if you do have a spare, you still get the benefit of a clearer beer. Over time, the hops will give the beer a very strong grassy taste if left in, so the gelatin is helping out here also. (I've left 1/2oz of hops in a bag at the bottom of a keg - initially the flavor was great, but after a month the overpowering grassy taste made the beer undrinkable without diluting with something else.)
In future, as well as using a bag, tie a string to it so you can pull it out after a week or so. The string can be tied to either the gas dip tube, or simply hanging out the top of the lid - wedged between the O-ring and the keg mouth. If the string is thin it won't affect the seal.