magento2 – How to fix / core / index / notFound

I have been working on a Magento 2 development project for a few months now, and until recently there were no problems. When I started it today, my main css file came back as follows: and none of the main css css sites are working. The CSS for the other extensions still works, only the styles.css file on the main site does not work.

I cleared the cache, executed setup: upgrade, s: s: d, setup: di: compile, composer update and nothing worked to get rid of this error. No errors are also logged. I searched everywhere, but I couldn't find anything regarding this particular error, especially since it worked fine yesterday.