Clean it with bleach, leave the lid open for at least 24 hours. This has happened to me. My kids left my chest freezer open, nobody noticed till days later.

Second time in as many weeks I have come across unplugged freezer stories. One would thing that the electronics people know about an inexpensive warning device. I think I will ask around.

If you live where there is significant sunlight you might try moving it into the sunlight while attempting more ordinary cleaning methods.

Not sure if this will work in a freezer but it works great in rooms. Put a bowl of vinegar in the freezer, close the lid and leave it for at least a few hours. Vinegar absorbs odors. I do it regularly in the bathrooms, and sometimes in my teenage son's room. I also put some in the car to absorb smoke odor.

This will only help preventing future cleaning problems with unplugged freezers. The people on Gardenweb's Electronics Forum say that there are so called indoor/outdoor thermometers which have warnings features and can be purchased quite inexpensively.

Cold kills the odor, and time kills the smelly source.
Wash it well,, plug it in smelly or not. You wont smell it once it cools down and over time the source of the smell that you cant clean which is in the insulation areas will go away.

BTDT .. been there done that!

A mover suggested to my friend that she put a sock filled with fresh coffee grounds in the freezer and frige as they moved across country to prevent the smell.

This happened to me a few years ago. I went to my local pet shop and bought activated charchoal. l and used the coffee as well I just dumped it on cheap paper plates and in 3 days there was no smell.