What does the compiler say?

First check the Run window in IntelliJ and see are there any errors. (alt+4 to bring up the window if it’s hidden)

Can't compile or run at all?

Check did IntelliJ properly locate the previously installed JDK (File -> Project Structure -> Project SDK -> New… if neccessary … should say 1.8)

Compiles, actually runs, but something seems to be wrong anyway ?

Ask yourself: did you set up MySQL properly? Can the backend connect to the DB?