Radiator leaking

So I just purchased this van two weeks ago, previous owner said some work on the radiator was recently done. I can't remember what it was. Since I've owned it the temperature gauge really never goes up nor down very much but the heat worked fine.
Now suddenly the heat only blew hot when idling. I couldn't check the coolant at that moment because I was running late but on the way home my check engine light came on and temperature gauge was in the middle. Of course I pulled over but without letting it cool I couldn't check the coolant. I popped the hood to see if I could at least see any in the reservoir. Looked pretty dry so I called a friend who told me it would be okay to pour some directly into the reservoir. Where it leaked into the ground as I poured. I was later able to open the radiator cap, start my van (with cap off) and fill it. What is wrong? Thermostat? Or something more severe?