What is wrong when your refrigerator isn't getting cold enough?

A few days ago, I opened the door of our side by side refrigerator and noticed a lot of frost all over everything. I thought someone didn't shut the door tight enough. I made sure the doors were shut. The frost went away but now the refrigerator isn't getting cold enough. I found ice in the refrigerator airflow vent this morning, broke it off, and raised the cool level a notch. It's been a few hours but it hasn't seemed to have cooled down. What should I check? What do you think is wrong? The freezer seems to be working fine now and no more frost there.