I have not heard back from you, so I wanted to give you additional information to help open your safe. If the key you have is round and hollow, put that in the keyhole, turn to pop out the cylinder, and then REMOVE the key. Then you can enter your combination on the electronic key pad.
Please check to make sure your batteries are installed correctly and are alkaline. If the above doesn't work, try another set of batteries, because even though they're new, they may have been sitting on a manufacturer's shelf for a while and this drained their power.
If you're positive about your combination, that's great! If you have any doubt, you can contact Sentry and request your factory set original combination. Please visit the site below and follow all instructions:
Please note that their Customer Service phone number is ***** listed on that page, if you need to call.
If it is online, I could send you a downloadable manual for your safe if you can find and provide me with the model number. You will need both the model and serial number to fill out the lost combo or any form, to get help from Sentry.
If you're sure of your combination and it has not been changed since you own the safe, you can use your key, enter your combo and turn down the handle within 4 seconds. Also, don't allow more than 5 seconds to elapse in between number entries on the panel.
The rubber gasket in the door may be stuck, due to humidity, as it swells in warm places and from age. You can take a rubber mallet or hammer with a towel wrapped around the head and hit the door above and below the keypad, then all around the edges of the door, concentrating on the upper left area. In addition, you can tilt the safe to the sides and front to back to dislodge any item inside that may be blocking the opening mechanism.
I hope you are able to open your safe and if not, call Sentry to troubleshoot, further.
Please let me know if you found my suggestions and answer helpful and rate positively so I am credited by the website for my assistance. Thanks! If not, do not rate negatively, but please click 'Reply' and I will be more than glad to continue our conversation.