What Causes Ringing In Ear?

Hey i have ringing in my left ear. i had two bad teeth on the bottom left so i had them removed. The ring stoped after 24 hrs after that but no when i lay down the ringing in my left ear starts backand even when i stand up it still ringing then in about 2 hrs it ll stop if i lay back down it wil start back. I have had a mri done and they said everything looked fine. What could it be.