my iphone internet is not working-HELP?

[LouiseUmpelby]

Ok so last night i was on call and the work phone was running very low on battery, I didn't have a charger, so both phones being on O2 i swapped the sim cards until i had finished working. The work sim in contracted and my personal one is PAYG. The sim worked fine but when i put my own sim back in the internet wont work at all. Ive tried resetting my settings to prompt it to work but having no luck. Anyone know what is wrong??? I don't know what to do. please help
Thanks in advance