My canon rebel xti will not turn on.

Such symptom indicate a malfunction o the DC/DC voltage converter circuitry, and the camera has two of them located in different boards.

In order to perform a correct diagnostics and repair of it, it is necessary to open such camera and perform it by a qualified camera repair technician.

If it's under warranty, I do recommend you to send it back to make valid the warranty.

If not, then you hav the options of pay for its repair, or do it yourself if you are willing tgo open it and perform some basic diagnostics of the failure. You will needtools to do it, which includes the usage of a soldering iron.

Best regards,