Freezer Not Cold

Faulty thermostat

My DH is a refrigeration tech--he chimes in here with a likely Freon leak. As others have already mentioned, you don't want that leaking into your home environment.

If it were me ( and it already has been--we had a freezer about 12 years old that we just got rid of)--if it's a decade old, or older it may be in your best interests to dispose of it. The price of the tech call may be worth more than the unit.

Also, once the leak's been repaired--if you go that route--there will now be a repaired spot that will be possibly more prone to failure in the future.

We also found that once the new freezer was in, our electric bill dropped. The older units are far less energy efficient.

I, too, was just going to say make sure there's not a freon leak. Also, you simply might need more freon put in. Same goes for your car's air conditioning.

How old is it? Does it have freon, the cooling agent? If it is over ten years may need top up and to be checked that it is not leaking (you do not want fsc's in your home).
Unfortunately as a tradesman myself appliances need their repair specialists. You can often find good ones for not too much money at the parts stores. If you have an old fridge many counties have eco friendly buy back programs to help offset the price of a new fridge.
If your fridge is not old check your settings first before any phone calls. I have a friend that repeatedly bumps mine every time he puts beer into my fridge. Check the simple stuff if you haven't already.

Call a repair person to check it out, good luck.