Apps not loading is not related to changing SIMs or unlocking. Something else must've occured.
Did you update the version of iOS at the same time too, or restore the phone as part of the unlock process?
If so, you may be experiencing a common issue that happened after an iOS update some time ago related to AppleID authorization.
Try downloading one (free) app from the AppStore (doesn't matter what) and install it. Quite possibly, after doing that, you'll find all your other apps magically start working again.
See Step 3 here: http://support.apple.com/kb/TS1702 for more information.
have you checked that the new sim settings has not deleted or re-set all your handset settings?
( -on the asumption that you can still connect to the internet...)
(or) that it has deleted your account and/or password.
was it unlocked by 02 ?
(it must be unlocked by the 'official' ORIGIONAL network provider (or at an APPLE STORE);
this prevents unlawful, ie stolen handsets being of any use. The same goes for 'unofficial' or 'jail broken' handsets.)
if not it could be that the phone is not legitimate, and is now being blocked.
Thank you all so much,
It looks like I was wrong about the sim change, it was just bad timeing.
I have tried to download other Apps but with the same result, they seem to be on the phone but quickly return to the home screen and nothing more happens. The Apps that come with the phone are working fine.
The main reason I bought this phone was be able to have europeon maps, Tom Tom looked the best, and haveing them on my phone would be far better than a Sat Nav in the car.
Is there anything that I do to sort this out, I know that the phone is old and I have no one to take it back to, is this the end of my iphone adventure.
Once again, Thank you all for your help