Text messages stuck open

Force the Messages app to quit: How to force an app to quit on your Mac - Apple Support

SV6166 Said:

"Text messages stuck open: My text messages are stuck open and I can't read, send or receive. I have a new MacBook Air. How do I get them to close out and actually work"

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Closing a "Not Responding" App:

1. Exit this App:

Use the following to exit the app and get to Finder the (Smileyface). Use My User Tip: [command+tab] Use it to Access your Mac when Something is Not Responding - User Tip

then...

2. Force Quit this and All Apps:

1. Go to: Apple menu
2. Select: Force Quit...
3. Select: an app
4. Click: the Force Quit... button
5. Click: the Force Quit... Button.
6. Perform seeps 3-6 for all listed apps
7. Restart your Mac