Basically, you're inside a tightly sealed, extremely cold, giant metal box. You need to worry about: hypothermia ,frostbite and air supply.
In order to keep hypothermia（体温降低） at bay, you need to maintain your core body temperature. Your best bet at doing this is by fashioning some kind of protection from the cold.
You lose body heat in a number of ways. You lose heat when you breathe and perspire. Large areas of exposed skin radiate a lot of heat. Heat can also be conducted from your body through contact with cold surfaces such as snow, or in this case extremely cold metal. Obviously, you have a limited cache of useful supplies, but all is not lost. You've gone into the walk-in to repair some shelves, so you have a roll of duct tape in your pocket and an all-purpose tool like a Leatherman or Bucktool.
To inhibit frostbite, you need to make sure that your extremities are covered and protected from the cold. The plastic tent or suit should help with this. Your head radiates an incredible amount of heat away from your body. So, if your t-shirt is long enough, cut off any extra material from the hem, making sure not to expose any skin, and use the material and some duct tape to make a hat or head wrap and a pair of mittens.
Now that you know what to do about hypothermia and frostbite, what about the air? Let's say you're in a freezer that is 20 by 10 by eight feet, and it's completely sealed. That means you have 1,600 cubic feet of air to breathe. Initially the air is 20 percent oxygen and nearly 0 percent carbon dioxide. Each time you breathe, your body consumes oxygen and releases carbon dioxide. You inhale air that's 20 percent oxygen and 0 percent carbon dioxide and exhale air that's about 15 percent oxygen and 5 percent carbon dioxide.
A person at rest breathes about 2,800 cubic feet of air per day. If you do the math, you'll see that a person needs about 150 cubic feet of pure oxygen per day. There's 320 cubic feet of pure oxygen in the freezer. People are OK with oxygen concentrations down to 10 percent or so, so there's enough oxygen to last for about a full day in a freezer this size. No running and jumping around however -- oxygen is precious in an environment like this.
Let's say that you're successful with your tent, hat and mittens. When the morning crew arrives almost six hours later, you'll probably be flushed, weak, and dizzy or disoriented from the carbon dioxide. Also, at best, you'll almost certainly be suffering from mild hypothermia, so your speech may be slow and you'll have limited control of your hands. You will need fresh air -- perhaps even supplemental oxygen -- and treatment for hypothermia. Even if you aren't exhibiting all of these symptoms, it's probably best to seek medical attention.