From a search on the Web: (Ask How)
"Have the unit serviced if "H," "r," "S," "AC rE," "dc rE," "AC HE," "dc HE" or "Sr" appears on the display. These errors are not serviceable by the owner."
Sorry for those bad words. You can try unplugging the unit for a couple minutes, plugging it back in and see if that helps any.
I'm sure you tried a simple turning it off and back on again.