I have a Galaxy SIII (Android 4.3) phone on the Vodafone network (UK). I was using WiFi when available and mobile data occasionally. It all worked fine. When on mobile data, I would see an icon on the top bar showing a little letter H with two arrows underneath.
But now the mobile data has sort-of stopped working. I say sort of because it appears that I can still turn mobile data on, and I still get the little H with two arrows, but none of my apps that use data appear to actually be able to communicate with the internet. I don't actually get any "no connection" messages, instead, I just get rotating wheels and other indicators that they're trying to get something that's not arriving.
If I switch back to wifi then everything starts working again.
EDIT: I have not installed any custom ROM. The phone is not rooted.
EDIT: I just called vodafone, who suggested a variety of things that made no difference. Now they've said it may be a fault in my SIM card and are sending me a new one.
EDIT: New sim card arrived. No difference. More calls to vodafone, multiple suggestions... nothing has worked.
EDIT: Tried sim in Galaxy SII... all symptoms identical. Saw little H icon (sometimes flipping to "H+" or "3G"), but no mobile data getting through.
SOLVED! it suddenly started working again. I suspect it was a fault at vodafone's end and my last call to tech support induced someone there to correct the fault.