New freezer not cooling - temp @ 25 degrees and food is defrosted

We have a new freezer (less than 6 months old) - model #FKFH21FHW. Twice in the last 3 weeks the freezer has failed to cool. We removed all the food the first time and unplugged the unit for several hours. All seemed okay after plugging it back in and the temp stayed at 0 degrees for the last three weeks.We started putting food back in within the last 48 hours. Now...the temp is rising agan (25 degrees) and all the food is defrosted. The door is opened 2-3 times per day for 3-4 seconds (max) at a time. What could be wrong?