My data is turned on. I don't have any limit set.
When this happens, I have to turn my data off and then turn it on again.
I also notice that this happens a lot when I'm moving around.
For example, if I'm at home using wifi and then when I go out, it just get stuck with grey bars and then I have to turn my mobile data off and then turn it on again before I actually get a data connection. This also happens when I'm at one place (no wifi) just using data connection, and then when I go to another place, it then get stuck with grey bars.
When I'm at one place, this doesn't happen.