Can't open System Preferences

Try starting your mac in Safe Mode, How to use safe mode on your Mac – Apple Support

Safe Mode takes along time to boot up, so be patient, then try opening System Preferences.

Then restart your mac as normal and see if System Preferences opens.

If that doesn't work then you will probably have to reinstall macOS High Sierra.

If you don't already have a copy of the Install macOS High Sierra.app

then you can download a new copy from here, open your Safari browser,

Firefox will not work.

How to upgrade to macOS High Sierra - Apple Support

Go to Section 4 and click on Get macOS High Sierra

this will redirect you to the App Store High Sierra page

it should say Get or Download. This will download the

Install macOS High Sierra.app to your Applications folder.

Connect your mac to your router via cable rather than rely on WiFi.

Do not use your mac while it is downloading

and do not use any other devices that share the same internet connection.

Before reinstalling make a full back up of your Mac.

You can just install over what you have your data will be fine

there is no need to wipe your mac and reinstall that way.

Once you have downloaded Install macOS High Sierra.app make a copy of it

and keep it safe on an external drive, the one in the Applications folder self

deletes at the end of the install. Having copy is handy in case of times like these

and saves you having to rely on Apples Recovery Servers or downloading again.

If the App Store says ‘Open’ then it is detecting a previous download of the install app

go to the Applications folder locate Install macOS High Sierra.app.

Move it to the Trash, empty the Trash.

Restart, click on the link again, the App Store should say ‘Get' or 'Download'.

have you tried a restart of your mac and then tried opening System Preferences.