IntelliJ gives Fatal Error: Unable to find package java.lang in classpath or bootclasspath

Somehow the JDK language jars were removed from the classpath. All I had to do was reapply the JSDK home path.

Goto:
File -> Project Structure -> Platform Settings -> SDKs
Re-apply the JSDK home path.

Doing this added about 15 jars to the classpath. Apparently these are important for compiling.