Yesterday, iOS 8.0.1 showed up and turned a whole bunch of new iPhones into iPod Touches. If you installed the bad update and lost your mobile service, there's good news and bad news. A fix is on the way, but it might take a few days for Apple to clean this all up.
From Apple's support page on the subject, which also walks you through the revert-to-iOS-8 process:
We are also preparing iOS 8.0.2 with a fix for the issue, and will release it as soon as it's ready in the next few days.
With any luck that could be today or tomorrow, but Apple's clearly building in room for a bigger gap.