You have forgotten your lock combination.
Call the lock manufacturers. Most of them keep a record of the lock combinations for just such an emergency. However if it is one of the really secure safes, then even they will not be able to help you open it without breaking the safe open.
Hence keep the combination number safe somewhere, and once you have had to check it, change it for a new one.
You are unable to open your lockset to repair it.
If your lockset consists only of a knob, then look below the knob. There will be a small hole there which will house the screw to open the knob. Most other locksets will have visible screws to open them
Your lock seems to have frozen in the cold weather.
Try to heat the key gently and handling it with gloves put it inside the lock again and again. This will help to deice the lock. You can also try to use a hair dryer to heat the lock. There are also commercial deicers available in the market and if you live in a cold area, keep one handy.
There are many other problems that can happen to locks, but before you call the locksmith, do try some home remedies.