Canon Rebel T2i EOS 550D Instruction Manual page 219

The battery becomes exhausted quickly.
Use a fully-charged battery pack (p.24).
The rechargeable battery pack performance will degrade over
repeated use. Purchase a new one.
If you use Live View shooting or shoot movies for a prolonged period
(p.107, 123), the number of possible shots will decrease.
The camera turns off by itself.
Auto power off is in effect. If you do not want auto power off to take
effect, set [5 Auto power off] to [Off].
Even if [5 Auto power off] has been set to [Off], the LCD monitor will
still turn off after the camera is idle for 30 min. Press the <B>
button to turn on the LCD monitor.
Shooting-Related Problems
No images can be shot nor recorded.
The card is not properly inserted (p.31).
If the card is full, replace the card or delete unnecessary images to
make room (p.31, 171).
If you try to focus in One-Shot AF mode while the focus confirmation
light <o> in the viewfinder blinks, a picture cannot be taken. Press the
shutter button halfway again to focus, or focus manually (p.37, 69).
Slide the card's write-protect switch to the Write/Erase setting (p.31).
The image is out of focus.
Set the lens focus mode switch to <AF> (p.33).
To prevent camera shake, press the shutter button gently (p.36, 37).
If the lens has an Image Stabilizer, set the IS switch to <1>.
Troubleshooting Guide
219