Why isn't my Minecraft server working anymore?

I recently made a minecraft server for me and my friend (about a month ago) and it's been working great Barely any lag, both of us have been able to access it fine. On Saturday, my friend tried to go on it and it wouldn't connect. I tried to go on it and it was fine. Me and my friend have both tried many things to allow her to go on it but nothing seems to be working:/
Any help on what I could do?