My Canon EOS rebel suddenly won't take pictures in dim light. The flash pops up and blinks and blinks trying to register the image but won't ever autofocus and take the picture. The light has to be very bright for it to take a picture. I have gone through all of the settings and everything seems to be correct. Any thoughts?

The 'fluttering' is a pre flash used to help the camera focus in low light. The internal flash isn't a powerful flash and is usually only good enough for subjects up to about 2m away in reasonable light also to light a subject that is back lit. Try putting the camera in manual mode (M) 1/60sec on f5.6 flash popped up review it and make adjustments accordingly i.e. open up or shut down the aperture as the flash usually syncs at 1/60

Does the flash pop up when you try to take a picture in low light in the Green Box Mode? If the flash does not pop up the problem may just be that something sticky is keeping it down. If it pops up but does not fire, the problem is usually either a bad flash tube or the wiring to the flash has broken where the flash head pivots up and down.
When the flash is fully charged the capacitor retains a voltage of over 300 volts even when the camera is turned off and the battery is removed. I've seen several cameras where the owner tried to repair the flash and ended up burning out the entire Main PC Board.
Bob Kilbourn www.camerarepairs.com

It's probably too dark and the flash isn't powerful enough to assist the focus mechanism on far away objects (more than 10-15 feet).

If you will need to switch to manual focus and maybe manual exposure.

Will it not pop up under ANY circumstances? Try putting the camera in Manual (M) mode, set the shutter speed to 1/200 and the aperture to F5.0, then go in a dim room. Press the button on the camera body with the flash symbol on it. Does it still not come up?

Frustrating I am sure. I had the same problem on my first XTi. I cannot remember what the settings were, but I think I was set for portrait and had flash set as the white balance default source. Therefore the camera always popped up the flash assuming that I wanted or needed fill flash.

Change you white balance to sun/shade/tungston etc as is appropriate, choose a faster ISO speed in lower light levels and try using the P or Tv mode and use a shutter speed that generates a middle range aperature for your particular lens.

Flash Tube is bad inside the Flah head, tccamerarepair.com

Outdoor lighting covers a great many conditions...
from candles to bright sun on snow. It depends on the light, and you did not say much about the light.
Indoor can also vary quite a bit. I suggest you get a good flash that tilts up and swivels around. READ the information packed with the flash. Note the coverage for each different ASA setting. This is very important.
Don't expect your pop up flash to light up a football field...it won't.
The correct settings are a balance of many things, including ambient light, film speed, surroundings, and intended use of the image.
Your owner's manual, local camera club, experience and library books will help you.

Move the mode dial to the no flash setting (full clockwise position)which is basically full auto with no flash (page 47 of manual)