Which Rebel camera is it? How long have you had it?
With the camera powered on, will the flash pop up when you press the button on the front next to the flash that makes the flash pop up? It is a lightening bolt symbol.
You may have turned the flash off.
Dial: There is one setting on the dial that is no flash. Right between the Green Box setting and the CA setting on my Rebel. Looks like a lightening bolt with a strikeout through it. Make sure the dial is not on that setting.
Menus: If it was not that simple, then it could be something in the menus. Rather than trying to trace it down, just reset all the settings. Go to MENU (press "MENU" button). One of the YELLOW menu pages has an operation called "CLEAR SETTINGS". Try clearing all settings. That is actually a good way to undo all kinds of mysterious little goof-ups.
Does that do anything?