I am really new to using kegs and gas. I forgot to gas the keg and 1 week later attached the gas but the beer will not carbonate. Taste is good and puts a head on when poured which lasts for a while but the beer tastes flat as it is not carbonated, HELP.
This is one of the great benefits you get when kegging. I usually keg my beers and store them for a week or two in my root cellar (@ 50-60ºF) before carbonating. I don't usually filter my final product so I find it beneficial to give the beer a little time to settle out some of the sediment that gets stirred up during transfer. It's also important to note that introducing gas during this window won't have any effect on the sediment settling out, but I personally notice a positive difference in taste when I wait to carbonate.
So to answer your question, you can indeed still carbonate this batch by force carbonating as outlined in Nhon Ha's answer, or just hook up the gas line and set it at 20psi for about a week (pull a test after about four days to see the progress). When you have the gas line hooked up and turned on, double check that the keg isn't leaking by using a soapy sponge to apply some suds to the connection points, main lid, and stress points. If you find bubbles at any point, turn off the gas and try to fix the leaks before continuing to carbonate.
One more note: Make sure your gassing at a colder temperature if possible. This will help the CO2 permeate the solution much more efficiently. Here's a helpful chart for determining proper carbonation levels for your kegging adventures:
You can still carbonate your beer. Put the gas into the keg, and shake the keg for 20mins or so until you stop hearing the gas going in. It then becomes carbonated. Best to put the keg on the side on the floor or table, and roll it back and forward.
I came across this article recently and found it useful. Thought I'd pass it on.