Wont show the screen

Check if your battery door is perfectly aligned and has no gaps if it has a gap then you have to push or tape it to line perfectly without any gaps and if the door perfect and still showing same thing you have change batteries make sure to use either Duracell or rechargeable 2.5v batteries

Getting out your battery is ok. Don't expect the lens to retract, because the camera has no power. But after you reinsert the battery, the lens should retract, because the camera knows in what position the lens is.
Be sure the battery is in good shape. If the battery is not fully charged or even worse, when it is defective, noting will happen. When you have a multi meter, you could check the voltage of the battery. It should read a little more (few tenths of a volt) when measured with a multi meter. if it is only slightly less then what is printed on the battery, you can assume the battery is defect. The camera will not work with a defective battery, so in that case you should replace the battery.

Your batteries are too low.
When you switch on the camera, the lense go in. Then lense comes out fully. Then, if the batteries are low, lense goes in and camera turns off.
But if the batteries are too low, camera turns off before the lense retracts or even before the lense comes out fully.

Check to make sure the batteries are inserted correctly.
Clean the battery terminals with a dry cotton swab that has a fine and soft tip, and then re-insert the batteries.If the [Memory Card / Battery Cover] is not securely closed, the power of the camera may not turn on.Close the [Memory Card / Battery Cover] securely, and then turn the power on.
If your camera still does not work even after you have performed the above procedures, your camera may be malfunctioning. Please have the camera serviced in this case.

Try to use fresh batteries, and push the barrel in and out when it turns on to see if the 'binding' of the focusing gear can go away when you do this manipulation of the barrel.

If the error continues, maybe the camera needs service by Canonusa.com.

I could not, unfortunately, find a reference for an a450 on the website, though. Is that an older model?

It looks pretty much as battery problem, try using a new and charged one. Low charged (bad)battery can do strange things to eletronics

I had the same problem that the green light turned on then the red one, even though the camera batteries where new.

I found a solution for this problem, hope it works for all as it did for me.

http://www.sendspace.com/file/ysolwv

The link shows the images of the steps you need to follow to fix this problem.
This problem actually is from low batteries or low voltage in the camera, but its not because the batteries are dead, it is because the batteries don't have a good contact with the metal conductors in the camera.

In the images you can see that the camera has a plastic cover for the battery contacts, so the way to fix it is to remove that plastic cover with a small screwdriver, and pop in the new batteries.

With that it should fix the problem. Hope it works.

i had a similar problem...my lense woudnt go back in either, but it kept saying low battery and bleeping. I realised that my lense was stuck skwint, and that was what was causing the problem, once i straigtend up my lense it folded bak into the camera and worked fine wen i turned my camera on :D

Most probably weak batteries (this can happen even if new batteries are used). Recommend rechargeable NiMH batteries for the A400. See the following:

http://camerarepair.blogspot.com/2007/11/simple-fix-for-dead-cameras-that-wont.html

the dead batteries affected on the camera