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It is likely that your safe needs lubrication. Not with grease, oil, nor WD40, which can each and any of them gum up the workings.
A safe lubricant, otherwise known as lock lubricant, can be found in a good hardware store. It is made of powdered silicone-based stuff, and is to be blown into the crack at one side of the door where the latches keep and hold the door shut.
I hope that helps... if not, your safe has a severer problem that is probably going to need either being broken open, or having a locksmith called in (possibly, to break it open himself.)
If you need further information, let me know, please, and I will continue to advise you.