Help on getting left menu back in PC settings

Settings is an app, supplied with Windows, and can be reinstalled. Perhaps that may help.

Reinstall and Re-register Apps in Windows 10 | Windows 10 Tutorials

Search that for Settings, follow the instructions at the top of the relevant section and use the powershell line given.

Note: you may wish to check if you need to do it for 1 user or all users if applicable.

You might also try placing your cursor on the very left edge of the settings window until it turns into a double headed arrow. Left click and hold it and drag it to the left until the side window appears.

@essenbe is correct- the appearance of Settings changes if you drag the left edge of the window, so that left pane suddenly appears/disappears. Make sure you drag far enough.

Please check again just in case. That seems the most likely thing to do.

Another thing to try is maximising the window. What do you see then?

Otherwise:

Please create a new user for test purposes. Does the same thing happen when logged in as that user?

If so, it's a general Windows problem. If not, it's specific to the user profile. Thanks.

History- when did this happen? Was Settings ok previously? Are you aware of any changes that may have led to this?

Do you have any System Restore points you can use?
Do you use disk imaging as most of us here strongly advocate - e.g. Macrium Reflect (free)?

That's ok, I don't think I've ever dragged it myself enough to do the vanishing trick, hence didn't think about it.