Dreaming of a tombstone can be a very ominous experience, but the tombstone alone should not be the only symbol considered when trying to understand its meaning. In most cases dreaming of a tombstone does not signify an actual death, but may symbolize an ending of some sorts in your life. So if you dream of this symbol, work on recalling as many details in the dream as you can.
Is there writing on the tombstone? This might give you clarity on the ending you are about to experience. Are there people with you in your dream? One of them may be on their way out of your life, but not in the physical sense. Maybe they will be moving away, or changing jobs. Maybe it will be you that will be moving away or changing jobs.
In some cases we need to see symbols of where we need to create endings in our lives in order to move forward, and a tombstone in your dream might be doing just that. Endings are never to be feared in our lives, as new beginnings are always opportunities to experience new miracles as well.