'Registering your computer on the network' problem

I use 3G Mobile Broadband with my ISP 3 and yesterday my internet connection just went off for no reason at all. I've had this before so I just went to reconnect again but in the process of: Opening Port>Dialing>Verifying Username and Password etc on the 'Connect To' screen, it won't go past 'Registering your computer on the network'. I've tried resetting my TCP/IP settings and this won't work and it's becoming really frustrating. It's never done this before and I have no clue how it's happened so if anyone could shed some light on this I'd really appreciate it. I use Vista Ultimate SP2 as well.

Thanks