When a Snapchat story turns grey, try logging out of the app and back in, if that doesn't help then try logging out, deleting the app and reinstalling.
Another thing that you could try before deleting, if you have an android phone, is to go into app manager and find Snapchat under downloads and clear data. You will need to enter your login details again but you should no longer be greyed out.
On an iphone double press the home button, find the app and swipe it away, this should reset it and hopefully it will be back to normal. Certainly worth a try before deleting and re-downloading. I hope that helps.
Please do remember when using Snapchat that nothing ever totally disappears into cyberspace and images that you send can come back to bite you when you are rich and famous :-)