If you can put your key in the trunk lock but it won't turn and it's very very cold out then chances are extremely high the tumblers inside the lock are frozen and dirty.
They need to be cleaned and then lubricated.
You can get lock de-icer, but all that is a few thumblesfull of rubbing alcohol in a five dollar pressurized can. You should be able to get a syringe of some sort from a pharmacy; just fill that with rubbing alcohol and squirt it in the lock. That should free it up immediately.
Now you need to lubricate it; you'll need dry graphite lubricant. This usually comes in a spray can with naptha and acetone as a carrier fluid that evaporates very quickly. Spray a good amount of it in the lock and it should now feel as smooth as it was when it was new.
Your key will come out fairly black for a while because of the graphite, so just wipe it off. It'll do this for a while but will eventually stop.
It's a good idea to clean and lubricate your trunk lock every fall; if you do this you may not get stuck-trunk-syndrome in the middle of winter.