Chest Freezer Issues

[rack04]

I have been using my craigslist chest freezer as a keg server for a little over 2 years. I have a temp controller set at ~45 degrees F. Yesterday I notice that it wasn't cooling lower than 58 degrees F. I opened the lid and noticed a lot of ice buildup in one area along the back but it was barely cold in all other places. I unplugged the chest freezer and I'm planning on letting it sit for a couple days. Any idea what could be the problem?