Help with 2002 Hyundai Santa Fe radio needed.

My wife was having trouble with the radio in her 2002 Santa Fe. As I was diagnosing the problem, I pressed the CD button while the radio was playing. The display now says “M1” and I can’t get it to go away. She says that this is going to drive her crazy, so I need to undo whatever I did. If anyone has the Owner’s manual, I’d appreciate some insight into this problem.
This is the stock CD/Cassette/Radio. She bought the car used, so she doesn’t have the owner’s manual.

Can’t you just disconnect the car battery to reset the stereo?

Maybe, but I don’t know if this stereo has a security code.
Also, I want to know explicitly how to resolve the problem, in case it happens again.

We got in the car yesterday, and I noticed that the radio was working correctly.
The “M1” is actually supposed to be “FM1,” so there must be something wrong with the display.

If you cycle through the other radio inputs (should be FM1, FM2 and AM) does the F/A still get cut off?

It did when it was acting up.
I’m not touching it again - I don’t want to get yelled at.