QuickLook stopped working
It is possible that some QL plugins are broken, you should check 

~/Library/QuickLook/
/Library/QuickLook/
/System/Library/QuickLook/


for new QL plugins (check the date of the files). Maybe a program installed a new (broken) plugin? But some QL plugins are not inside the Library/QuickLook/ folders you can use

qlmanage -m


to list all QL plugins, with

qlmanage -m | grep -v com.apple


you see only plugins which are not from Apple.