Netbeans Run project button disabled

This problem happened to me several times before, I used to restart Netbeans and it works every time.

Based on my experience, this happen when you close Netbeans while there are errors in the code and the compile on save option is working.

To solve this problem, just delete the folder Build from the directory of your project folder, and then start Netbeans.

Disable "Compile on Save" and restart Netbeans.