Don't decide to close your doors at the first sign of financial hardship. Just because one source of funding stops coming in, it doesn't mean that things are hopeless. Try to find a different source of income, explore new avenues. There may be a supply of money that you've overlooked.
Schedules change. You may start a new job that doesn't allow you the same amount of free time. Maybe you decide to start a family. In most instances, there will be more than enough people able to fill your shoes. There are, however, non-profits run by a very small group or even just one person. In that case, if someone simply no longer has the time, the non-profit may need to shut down. Whatever the case may be, try to do everything you can to find a replacement. If your cause was worth working for to begin with, it's more than likely still going to provide a valuable service that would be missed if gone.
Maybe your clientele base gets so low that you feel the assistance you provide is no longer needed by the community you're in. Perhaps your organization was so successful that the problems you originally set out to fix no longer exist. Congratulations on a job well done. It could also be a demographic change. If you provide meals for elderly people, but over time the community you work in changes to a young family neighbourhood, your service will no longer be as useful. But if you move your operation to a different part of the city, you could find yourself with a brand new, and very grateful, clientele.
In any instance, be sure that that you've explored all of your options before you stop providing what is probably a valuable service.