Installed ios5 on iphone- lost 3G coverage!

[mountainhigh]

I did what Apple wanted and installed ios5 on my 3G iphone. I am with La Poste mobile in France. Now I have lost all internet access - I have reinstalled the codes given by La Poste and double-checked them, to no avail. I can make phone calls but not much else. The iphone still connects to Wifi in the house, but once this is disconnected there is no connection to the internet. Can anyone help please?
Many thanks in advance

 

[post/ergo]

Have you checked that in Settings>General>Network that 'Enable 3G' and 'Mobile Data' are on?