Minecraft wiki entry for Setting_up_a_server, associated text: Unless you set a static IP for the computer that is hosting the game, the internal IP address can change. This affects port forwarding rules, and can make them invalid. Each modem or router has a different way of setting a static IP address. You should refer to the manual for your device(s) or online documentation for further instruction.
If you are having players connect to your external IP, your external IP can change if you do not have a static IP from your internet service provider. Use a tool such as WanIP to periodically check on the external IP address. You may also search "my ip address" on Google and it will show your IP address. Alternatively, you can look into a DDNS service that will allow you to have a name, rather than an IP address, that will remain the same. The name will point to your external IP address, regardless of whether or not it changes (the DNS is updated when changes occur, hence "dynamic").
For troubleshooting purposes you can try running Minecraft on the server machine and connect locally. You can connect through either localhost, your home network IP (192.168.x.x) or your public (Internet) IP.
If for some reason you have trouble with connecting publicly over your IPv4, try connecting over IPv6. This should only be done for testing whether your server is online, external players should still use IPv4.