Can't host a Minecraft server anymore [duplicate]

Have you done any updates on your computer? Has your firewall/antivirus updated? Try and check your firewall settings and allow the games/ports for the games to see if that helps.

I experienced a similar problem over a lan game - I couldn't connect to any game that my friends laptops were hosting, nor could they connect to mine.

It turns out that they were running Java 6 whilst I'd updated to Java 7. I downgraded to Java 6 and suddenly I could connect. (A host of other problems were suddenly sorted as well, apparently Java 7 & Minecraft don't like each other)

It sounds like you didn't give your computer a static IP address when you port forwarded your router. When your internal IP address changes, no one else will be able to join your server because your router is sending their connection request to your old internal IP address.

Setting a static IP address is a moderately simple process. It is explained thoroughly with pictures on this page.