The easiest way to open a Sentry Safe without the key is by contacting the company to get a new key. If you cannot wait, you can insert the filing board of a nail clipper into the lock and jiggle it around a bit. This should open the lock.
Fireproof safes are designed to resist heat and smoke in the event of a home fire. Important papers such as birth certificates, passports and social security cards should be kept in such safes. However, these safes are not designed to resist burglary. They use thinner metal than other types of safes, and the locks are not particularly secure.