As above cool boxes are limited to the battery you use to power it. The car battery may not last overnight. A large leisure battery may last longer but 24 hours maybe its limit.
Does the fridge light? Thats about as much as you can check really. After that its a specialist job.
We had trouble with our fridge last year and took it to Stewart Langdon's and they were brilliant. Fridge fixed in a day and cost wasVERY reasonable. Would recommend them every time.