97 grand voyager engine temperature

We have a 97 Plymouth Grand Voyager, and today while driving it, the engine temperature light came on, with a ding, ding, ding, sound. Shut the engine off, and turn it back on, and it's fine for a little while. I'm in Sarasota, Florida, and it was a hot one today, so the A/C was on. The funny thing was that the gauge with the indicator needle seemed to register normal, it was just the light that would come on.

I looked in the coolant reservoir, and it was a little low, so I filled it back up again.

The problem continues. I hope it's something cheap. Any ideas, and how much should it cost? Is it something that a halfway handy fellow can do on his own with a manual in his own garage?

Thanks ever so kindly.

-Mark