sara sabeha, since you already tried all the buttons, and if there is not physical damage to the phone, I would see if you have your "manner mode" engaged. This mode will set your phone pretty much in silent. Here are the instructions from the manual which is available right here
" Using Manner, Silence All, Lock Mode
Manner Mode (Quick)
Use Manner Mode in a public place.
1. In Standby Mode, activate Manner Mode quickly by pressing
(for 3 seconds) instead of using the Menu function.
- When the cellcase is closed, Manner Mode can be activated by pressing and holding the Voice Memo Key. G When Manner Mode is set, Manner Mode( ) is displayed on the LCD screen.
G When Manner Mode is set, key tone does not sound and Vibration is selected as the ringer.
NOTE Selecting Manner Mode will allow you to maintain your phone under Manner Mode even when you turn it off and turn it back on.
NOTE To cancel Manner Mode, press again until Silence All activated! is displayed and press again until Normal Mode activated is displayed. By the way, here is the picture for you to identify which key it is :-) Good Luck and I hope this helps.