If you choose to not go to Equestria then nothing happens. There are no punishments or rewards for declining.  
If you choose to go to Equestria then the only condition is that you stay until you die. The cause of death does not matter.  
If you are incapable of death then after 1000 years you will enter a death condition and your host body will lack a mind, rendering it a vegetable. Otherwise cause of death does not matter.  
Once you die in Equestria you return to earth as you were at the time you left.  
Nothing is gained or lost outside of Equestria.