Help Data network won't turn back on

[joermungard]

I've just switched cell phone carriers, and as soon as I put in the new simcard, the data network turned itself off and won't turn back on (as in: I've already tried turning it on through the settings and activating packet data) - whenever I select "activate data network" from the menu that appears when you push the power button, nothing happens. I've tried using my old sim, but it didn't work there either. I've even tried a factory reset and it STILL won't turn back on.
Any ideas?