Error establishing a database connection

Hello,
I was following this tutorial on Udemy on how to work with wordpress, the instructor installed several plug-ins and so I followed him right until a certain point where I began facing this error whenever I tried refreshing the page while editing it.

Basically when I use this page builder plug-in and then insert any other widget inside it then attempt to refresh the page it shows me that error: “Error establishing a database connection”.

I’ve tried to save and refresh the page without using that plug-in and every time it didn’t show me the error message so I concluded that the issue must have to do with the plug-in.

I am sorry if I missed on something, I absolutely have no idea what I am doing since I just started using WordPress hours ago and I hope someone could direct me on what to do.

Thanks in advance