After installing iOS 7 you may be asked to provide a passcode; if you haven't ever defined one...
If you don't see the word "skip" in small blue letters try the following. Otherwise tap "skip".
Enter any number and remember what it is. See if this lets you in. If so, go to Settings>General>passcode lock. enter the number and then you can turn off passcode (a vary bad idea, as many have discovered when their devices were stolem) or turn off and then on using a different number.
If your device is disabled at this point...
Connect to your computer in recovery mode per the instructions in http://support.apple.com/kb/HT1212
You may need to do this more than once.