I'm just new at web programming and designing. I have made a website and been accessing it constantly since I've been checking its functionality. I've just finished doing the fixes earlier and revisited the site. Everything went well. But when i tried to access my site again an error showed. Refer to the attachment below.

What causes this error? Is this kind of error bad? Do i have to do something to fix this?

Sorry if i may sound silly or stupid... I am a complete noob at this. Thanks!