99 means battery problem. Take out the battery and run the camera on AC power. Do you still get the error? Do you recall how long the battery lasted before the error occurred? See if it lasts 30 minutes with the LCD on.
Batteries do go bad and 10 years is a long time for one. Have you ever replaced it? You can try batteries.com or Amazon for a new one if you have no error on AC only. You want a BP-511 or BP-512 replacement. BTW your camera seems to also be known as an EOS 300D.
http

/www.batteries.com/digital-camera/canon/eos-300d-battery_R-415883-202380.html