Should be a fan unit inside fridge part, usually in back inside, look for the vents, check that fan, Check to see if the pipes are cold, any sign of frost on them. Frost is ok means its getting cold, so could just be the fan. if it is auto defrost, while you have the section open check the htr element, if warm, could be its stuck on, so unit will not get cold. quick fix until fixed properly, is unplug htr. The unit will then over a time ice up, until the htr part is sorted out.
SD, April 2012