I flipped the breaker switch off, and back on, which controls my outdoor security light. Now it remains on all the time. Might you advise me how to resume proper operation--come on only when it detects someone? I believe it is a heat sensor.

Elcho

Leave it off for 10 seconds or more. That will clear it and put it back into sensor mode. Some brands stay in test mode for the first 5 minutes.
Find the brand and get the instructions online. It will explain everything.

Might also flip it back and forth a couple of times if that doesn't work.

Turning the power off and immediately back on is the procedure for making the light stay on full time- on the motion lights I have owned at least,
Likewise as stanfortyman has mentioned, off for ten seconds and back on is the procedure I have seen to go back to normal mode.