the "frozen" thing is sometimes seen if the power button is pressed repeatedly, or not pressed hard enough... Be sure to hit the P (park) button first, as that sometimes helps to clear it.
(You may also have inavertently gone into IG-ON or ACC mode - is READY on the display when frozen? Be sure to apply the brake pedal firmly when pressing Power.)
There used to be a Knowledge Base article on this over on priuschat.com , but it appears that the KB is down/gone.
I'm guessing that your second comment on parking is more likely:
"CAUTION: Shift the position into range [P] when parking. The
batteries will not charge if the transmission is in neutral [N]."
This message on the display is shown in the Owner's Manual, but is often missed... US 2004, Section 1- 2 "Features on New Toyota Vehicle," Toyota Hybrid System, Precautions for use, page 14, and briefly mentioned again on page 121 under the description of messages shown when the Master Warning Light is on.
However, the manual does not specifically state that the message comes on if you open the driver's door and the car is not in Park...
What the owner's manual does say:
<quote>
When the multi~Vinformation display shows this message, the master warning light (red triangle around a "!" ) comes on in the instrument cluster. Read the message and follow the instruction.
^A When you leave your vehicle, or stop or park for a while, put the hybrid transaxle in "P".
^A Charging is necessary. In "N", charging will not be applied. The electric generator operates in "P", "D" or "B". If you continue driving, put the hybrid transaxle in "D" or "B" and depress the accelerator pedal. Do not leave the hybrid transaxle in "N". When driving in traffic jam, operate in "D".
</quote>
Also on page 27 of the same manual, section 2-1 "Operation of Instruments and Controls," "Keys and Doors," there is a chart of the various kinds of beeps you may hear from your Prius. A continuous beep inside the vehicle means that the car is not in Park, and the driver's door is open and/or the smartkey is no longer in the vehicle.
I think that the warning was more intended that you do not forget to put your car in Park (forget the car is on in electric, and just get out, and then the car drives itself into something), than it is a warning that you forgot your door open when trying to drive off.