Hello. My name is***** can help you with your question.
Based on your description the safe as both a combination and a key lock. If you no longer have the key then whether you can get in on your own will depend on whether the safe is program to open with either the key or the combination or if it requires both. These safes could be set up either way so what I would suggest doing is getting the master combination from Sentry and trying that. They have a process in place by which you can do that. If you are able to access the cylinder lock, and you don't have the key, you can also get a replacement key from them. But it sounds like the key is not accessible. I will give you a link to the page where you can request the master combination.
That is the link to get the master combination. If you cannot access the lock cylinder, then you may need to contact the locksmith to open it.
Please let me know if you still have questions.