While dying your hair can be fun, sometimes it can be damaging. Depending on your reason for not coloring your hair anymore, there are several ways you can go back to your natural color.
1.) You had an allergic reaction to hair color, and can no longer dye it. In this case, you probably had an allergic reaction to PPD or hydrogen peroxide, which are found in virtually all hair colors, even professional hair colors you see in salons. You have two choices: let your hair color grow out naturally, or use henna that's a similar shade as your natural color. Be careful, though. Henna can react poorly with previously-dyed hair, so make sure the henna you get has NO PPD or hydrogen peroxide, and make sure it can be used over previously colored hair.
2.) You want to have a baby, and no longer want to use damaging chemicals. This is a good reason to stop dying your hair. Better safe than sorry, right? If your natural hair color is lighter than the color you dye it, you're going to want to lighten the dyed part to your natural color--which will probably involve bleach. Don't do this if you are already pregnant. Use bleach to lighten your hair slightly, and then use the longest-lasting dye you can find to color over it with your natural color. If your hair is dyed a color lighter than your natural color (which is more common), use a long-lasting dye to go to your natural color. Keep in mind that because you won't be dying it for nine months, you really can't afford to have your color fade, so this would be a good time to invest in a salon color, or go to a professional beauty supply store to find a color famous for its staying power. Satin, by Developlus, is great for this, and can be found online.
3.) Hair dye is damaging your hair too much. This is a common reason why people go back to their natural color. However, because your'e technically not forbidden from coloring your hair, you can use a demi-permanent color with PPD and hydrogen peroxide, which is less damaging than a permanent color. Use a demi-permanent color the shade of your natural color. The demi-permanent color will not permanently change the color of your roots, so it's a way to grow out your roots without making it look obvious. You might need to do it more than once, though, as demi-permanent colors wash out eventually. However, these colors are not damaging, so this shouldn't be too difficult. If you've dyed your hair dark, and you want to go back to your natural light color, use Color Oops to get rid of the dark shade (or Colorzap, if the dark shade is darker than medium brown). If the remaining color is too orange, dye over it with a demi-permanent shade similar to your natural color, and just wait on it. You can also go the route of going Ombre--just leave your hair be, and in a few months, it might look like a fashion statement!