Why is my iMac opening up to the MacOS Utilities window? Just started doing that yesterday when I connected to home wifi. iMac Mid 2011 (A1311) High Sierra 10.13.6. I

It's doing that because you no longer have an OS on the computer. You need to reinstall macOS using the Utility Menu.

Reinstall El Capitan or Later Without Erasing Drive

1. Restart the computer. Immediately, at or before the chime, hold down the Command and R keys until the Apple logo and progress bar appear. Wait until the Utility Menu appears.
2. Select Disk Utility and click on the Continue button.
3. Select the indented (usually, Macintosh HD) volume entry from the side list.
4. Click on the First Aid button in Disk Utility's toolbar. Wait until the Done button activates, then click on it.
5. Quit Disk Utility and return to the Utility Menu.
6. Select Install OS X and click on the Continue button.

Depends. Two hours on a really slow computer. One hour on a medium speed computer. 30 minutes on a fast one.

Took me 24 hours then failed at 99% done.

Yeah, but that was the one that got water-logged.

Ha, ha, no another dry one.