Did you setup a cut-off for the data in your "settings".  Go in your "settings" and then press on "Data Usage" and there you can check if you have restricted or establish a data cut-off on your phone.  For more info, check out this link : http://mysamsunggalaxys2.blogspot.ca/...

Just encountered the exact same issue with my S2x.  I thought it could be related to the JB upgrade performed last Friday but I remember using data afterward.  Today I stopped at the Koodo kiosk, talked to a rep about this.  He checked my phone, APN settings, installed apps.... He had me uninstall Avast mobile security, thinking it could be blocking something...no go. 

Then he told me, I see nothing wrong with your phone, can we do a factory reset? ( Sigh) Well, ok, didn't have anything backed-up... once reset, no data still.  I was getting impatient so took the phone back home. 

i switched to koodo about a week ago and was supposed to get 10 days of free unlimited mobile data use, but it has not worked since i got it.  there isnt a limit on it, it is on, restarted the phone, tried everything... still wont work. 
any suggestions?