This is a fairly common issue on older acoustic guitars, especially those that were built for lower tension strings and have been strung with modern, high tension strings. Or those that were built with marginal bracing and top thickness to begin with, even for their original strings.
It's hard to dictate a repair without having the instrument in hand. This can be as simple as switching to light(er) strings, or as complex as major structural modifications which may require hours of skilled labor.
In terms of steps you can take yourself, taking off the strings and leaving it for several days (or a week) can be a good start - if the bridge returns to level, you can re-string with lighter strings, adjust the truss rod if needed to get the relief correct (based on the string gauge change - you do not want to try to compensate for the bridge issue with the truss rod), and hope for the best.
If the guitar has changed environments recently, consider the impact humidity may have. Guitars that have lived in a humid environment but were recently brought to a less humid environment will often suffer from a sunk bridge like this, due to changes in the geometry of the bracing as the wood shrinks due to lower humidity. If you know that the humidity has changed recently, you can consider taking the strings off, storing the guitar in a case with a guitar humidifier (basically, a little device intended to slowly release humidity inside an instrument case), and check after a week or two - then restring the guitar and keep it stored with a humidifier for the future.
Other than that, you are best off taking it to a qualified repair tech who has experience with acoustic guitars in order to get their evaluation on the instrument. Sometimes these problems can be caused by structural failures within the instrument (the bridge itself, or bracing inside the guitar, may become unglued over time). Those issues are sometimes awkward to fix but usually not expensive (figure an hour or two of labor if you're lucky). In other cases, if the instrument was underbuilt to begin with, techs can install a "bridge doctor" or other reinforcement designed specifically to address a sunken bridge. The most severe cases will sometimes require a neck reset or changes to the internal bracing. The good news is, these reinforcements are usually effective and permanent. The bad news is, they can be expensive (hundreds of dollars and up) and they will usually impact the tone of the instrument.
Whether or not a given instrument is "worth it" to repair is really up to you, especially in the case of an instrument with sentimental value. Most repair techs would probably advise that it's not worth it to reset a neck or do major structural repairs on an average or lower cost instrument, but many Martins are in the price range that would make at least basic repairs worthwhile.
