Wirelessly posted (8700g: BlackBerry8700/4.1.0 Profile/MIDP-2.0 Configuration/CLDC-1.1 VendorID/100)
Your sim card could be loose or the contacts dirty, gently clean. And reinsert.
Do you have Blackberry Data plan for your data? If not you won't be able to use the browser, email, etc.
the nextel logo will remain the startup screen regaless of your service carrier, you can't change that.