Motorola Photon 4G :: Turn Silent With No Reason?

From the last 3 days, after 3, 4pm, my photon turn silent, apears an icon with a bell and a red sign in front of it when i unlock de screen, when the screen is locked the bell that we move to the left to make the phone just vibrate has a bell with a line crossing in it, showing that it is silent. i looked in every place after clicked in Settings but i could not find some function that i changed, except the network configs that i move from automatic to gsm/umts only, to save more battery