If you didn't select Regina as your default launcher a simple reboot would suffice. If you did you will have to get into settings - applications - manage applications - Regina - clear defaults. After this press the home button and select another launcher or reboot.
If you can't get anything on your screen try connecting it to the computer, removing the battery, anything you can think of.
As a last resort you could searching for the original firmware and flashing it from your computer but you'll lose everything you can't backup from the computer including installed apps and their data.
