Q: i can nnot hear via my phone unless it ison speaker, how do i rectify this problem?

I am using iphone 4s for some time now, however today i noted i am not able to hear via my earpiece unless on loud speaker, i tried using the headsets too but to no avail. Moreover, i got a notice on the phone saying that headset mode is turned on, i have been trying to figure out how to switch it of but i am not. assist me to have this rectified.