Here's a thought if you have not been to the VZ store already. Go to Menu>Settings>About phone (at bottom of the list), and then press System updates at the top of that list of items. Does it say, "Your system is currently up to date?" If so that's okay otherwise it might start a system download. If it starts a system download then you have found the problem with regard to the message you locked on your screen.
OTOH, if your phone is completely locked up you were right in the first place, do not pass GO but go directly to the Verizon store.
This can happen if you push and hold the volume down button for a few seconds while the phone is powered down and plugged into the wall charger. Usually, removing the battery will get you out of that mode.
Since that doesn't seem to be working for you, you are probably best off taking it back to VZW.
On a different official VZ forum one person said that after being stuck in download mode they turned the phone off, removed the battery, and removed the SD card. After putting it all back together and restarting the phone it was okay again.