"Check Coolant Level" message appeared 2 weeks ago.Now no heat or A/C. Whats up? on 2005 Cadillac DeVille

Now the messsage comes on everytime the vehicle is started and runs for several minutes. The message can be cleared with the function switch and does not come back on while driving regardless of distance. Cooland in radiator and resevoir are OK. No engine overheating indicators. Car runs fine. Except no heat or A/C in cabin.

Asked by for the 2005 Cadillac DeVille