The warning sign informs you that you are in the Download mode. Try pressing one of these combinations while the phone is turned off:
- Volume Up + Power Button < standard recovery access procedure
- Volume Up + Volume Down + Power button < recovery menu for various phones (usually Galaxy Y Duos)
- Volume UP + Menu + Power Button < recovery menu for Android 2.3 based phones.
- Volume Up + Volume Down + Power button + Menu < recovery access for a very limited number of phones.
Do not press Volume Down + Power Button, because you will access the Download mode which is something different.