With your safe open and the lock discs visible (remove the door backing and any underlying enclosures), spin the combination watching where the "sweet spot" slot aligns for each number. You may see that the original combination numbers no longer perfectly align with the slot required for opening. Then, again, spin the combination sequence, carefully noting the specific number where the slots align perfectly, noting those new numbers. Finally, spin the combination using your new observed combination numbers. If those new numbers function better, that is your correct combination. However, if the original combination originally worked flawlessly then over time began to loose precision, your discs may be less than tight and slipping with use, resulting in the wandering combination numbers. Troubleshoot the disc tightness to avoid another recurrence of combination wander. There are videos on YouTube ....specifically for the cheap Sentry brand mechanical combination safes......demonstrating the technique to determine correct accurate combination. Good luck. Also, failing success per above suggestion....or first......call S&G. Their customer service is excellent.
Side note regarding electronic digital locks. There have been numerous concerns whether electronic digital locks would become inoperative, with safe contents inaccessible, in the event of an EMP event......natural or manmade. S&G has advised that even their earlier electronic locks were subsequently found to be EMP proof/resistant. Newer locks are marketed toward that EMP tolerant feature.
FYI, regarding what is an EMP event. From the internet for brevity.......An electromagnetic pulse (EMP), also sometimes called a transient electromagnetic disturbance, is a short burst of electromagnetic energy. Such a pulse's origin may be a natural occurrence or human-made and can occur as a radiated, electric, or magnetic field or a conducted electric current, depending on the source. Of sufficient impact, an EMP would burn out any unprotected electronic devices.....your car's electronic ignition, computers, flat screen monitors/TVs, smartphones, gas stations pumping systems, banking ATMs, anything with electronic circuitry.