using mobile broadband with a dongle

I am using a 3g dongle to access internet. But only a one time I could connect to the internet by using it. I followed the step that I have followed when I could successfully connect to the internet but it seems don't work again. the steps that i followed are first adding a new connection type as mobile broadband and selecting the service provider and other things. after that i even changed the APN manually. But it seems doesn't work like before. I am using ubuntu. and when i try to connect via using a wifi router it seems all are ok. how can i fix this error ?