There are some huge problems with your class that you have to address:
You user interface consists of a lot of static fields. Don't use static fields unless they are constants. Your application becomes very brittle: What happens when you create a second instance of your class?You are mixing different applications layers together in one class. It acts as both a controller, view and persistence layer. Like using static fields, when your application becomes bigger the code will turn into spaghetti and it will become unmaintainable.Your method names are not verbs. A method name should reflect what it does.Your user interface is not being built on the event dispatch thread. While this rarely leads to problems, subtle issues may arise which are hard to debug.You are not using layout managers. Layout managers are at the core of Swing. Not using them is asking for trouble. I wouldn't be surprised if this is what causes your issue.You are calling the setVisible() method from your class' constructor. This method is able to start new threads and you should never spawn new processes from a constructor.You're using a MouseAdapter to handle what should be handled by a FocusListener and an ActionListener.
First, split your class in one that just displays the user interface, and one that handles interactions with the database. Make them final, because there's not a good reason for them to be extensible. Remove static modifiers so that all fields are part of separate instances. Rename your methods so they express what they do. Rewrite the UI to use layout managers, in this particular case you may want to use a
CardLayout to switch to a welcome screen after the user has authenticated. Use a FocusListener to change the text of your fields when they're empty, and use a AbstractActions to handle what happens when the user clicks the login and signup buttons.