Car heat gauge going over midline where it's never gone

[Dennis S]

2002 Buick Park Avenue. The heat gauge has always been a little below the big midline, even when the temperature is in the high 90's. I noticed today, with a temperature of 67, that it's creeping over the line, which it has never done. It's too ho to look at the radiator right now. It's always right up to the lip when I check it. However, the overfill reservoir is dry. Can I add some distilled water to the reservoir? It rarely gets in the 20's here and will be awhile before it does. Of course, stuff like this always happens of Friday afternoon.