Good morning and welcome to Just Answer,
Your question is very common on our site as missing combinations are asked about on a almost daily basis. The recommended way in which to retrieve the combination is to contact the safe manufacturer directly as each combination is unique to each safe (this is for security reasons). Most safe owners who ask own one from Sentry as here in this link is how you can retrieve the combination from the company:
The safe company is going to be the only one who will have your unique combination on-file. This is for security reasons as if anybody online on the internet had your combination, then they could if they wanted to break in to your home and open your safe.
If your safe is not Sentry, then please let me know and I can retrieve the info for that company.
With following these steps, you will then be able to open your safe again.
I hope that the information is found helpful as please let me know of any additional questions/concerns you have. Also if you could take a second to rate my assistance so that I know my help was useful today.