This is one of the most frequently asked questions we get at Just Answer. So, you aren't the only one this has ever happened to.
Of course, you can always call a locksmith, but that is very expensive and often ruins your fire safe.
Luckily, Sentry has one of the best Customer Care departments in the business. You can contact them and they will send you replacement keys. Their telephone number is ***-***-****. Their web site is http://www.sentrysafe.com/CustomerCare/
It typically takes a couple of weeks to receive the new keys, but they are great about sending them, and it preserves your safe intact.
All the best,