Here's a list of things which may or not help:
- Make sure the IP you enter is correct. Try both adding a new server and direct connecting to it.
- Restart your Minecraft if you haven't already.
- Try using a different version (1.8 if you use 1.8.1 or visa versa)
- Re-download Minecraft, including the .minecraft directory. Back up anything you'd like to keep!
- Restart your internet connection/router, on your computer or physically.
- Using Command Prompt (for Windows), type "ipconfig /flushdns" and press enter. This flushes out and reloads your DNS.
If there's no avail beyond this point, I'm not sure what else can be done. Best of luck with your issue!