IDEA suddenly stopped detecting the JDK

I've been working on Java projects on IDEA for a while. Yesterday it suddenly stopped detecting the installed JDK.

The errors show up like so: https://i.imgur.com/UEXb1fM.jpg

As you can see, the project settings are set properly and I have not changed anything at all: https://i.imgur.com/elTH9QT.jpg

As a final comment, I re-installed Netbeans to test if this was a problem with IDEA or the actual JDK (even though there were no changes to it) and it works fine there.