Is there a fix for space bar preview feature not working any longer?

Try This:

1. Open the Activity Monitor on your Mac device. You’ll be able to do so by using the CMD + Spacebar shortcut

and typing “Activity”. Head over to Applications -> Utilities -> Activity Monitor.

2. Head over to Search (the magnifying glass icon located in the top right) and then type “Quick”.

3. Double Click “QuickLookUIService” from the search results.

4. It’ll open a new window, click on the Quit button located at the lower left.

5. You’ll get a pop-up asking you “Are you sure you want to quit this process?” You’ll need to click on “Force Quit”.

6. Close the Activity Monitor. With that, the Quick Look Preview issue will be fixed.

(You might have to log out of your computer and back in to take affect)

Follow previous advise First.

Fall back position if needed

1 - Restart in Safe Mode. This will perform a Disk Repair, clear cache files and only load Apple Software, extensions and fonts. The boot up will be slow and can take some time - Normal.

2 - Safe Mode will also eliminate Third Party Software, extensions and drivers from loading. It will only load the Minimum amount of Core Apple Processes to allow the the computer to function at a reduced Level of Performance 

3 - Does the issue present in this mode ?

4 - Sometimes a Safe Boot followed by a Normal Boot will just put things right.

Try restarting your Mac.

Booting into Safe Mode then restarting normally should sort it out.