Internet Connection - Mac?

So, recently I've been having troubles with my internet connection. I fiddled with the Diagnostics, and eventually the internet started working without failing every five minutes. I don't recall changing any settings, but it started using Airport instead of Ethernet. The connection seems to be slower now - downloads take much longer, for instance. At System Preferences > Network, it says Ethernet is not connected on the left and there's a red light instead of a green one, as there was before. It's telling me that the Ethernet cable is connected, but my computer does not have an IP address. How can this be, since up until a week or so ago I was using Ethernet? Also, is there a way to remedy this?