Apple - Quicklook not working

Try the following:

1. Open the Terminal application (/Applications/Utilities/Terminal.app)
2. Type qlmanage -r and hit enter
3. Type qlmanage -r cache and hit enter
4. Close Terminal and restart your system

This should force QuickLook to reload the generator lists and reset the disk cache.

Source: https://www.macissues.com/2014/03/22/fix-quicklook-errors-in-os-x/