First concert I went to was in a small room with big speakers blaring rock music. My ears weren't ringing when I came out, but when I started my car I couldn't tell if it was running or not without looking at the exhaust pipe.
Needless to say, after that I started putting cotton in my ears when I went to concerts. Honestly, I don't know why every rock band out there feels the need to be so loud. After a certain point, you can't distinguish the instruments anymore or even really hear their voice.
Edit: Oh, forgot to answer your question.

When sound waves enter your ears, they cause tiny hairs to vibrate. When prolonged loud noise enters your ears, your ear does something to reduce the hair's sensitivity to vibration to prevent nerve damage. When the noise is over, everything sounds muffled because the hairs are no longer as sensitive to vibration. If that happens, you've lost some hearing ability. Usually it won't be much, but if it happens a lot, it'll compound over time. If you want to know what causes ringing in the ears look up tinnitis.