Power source: power adapter, battery is not charging

I just bought my Mac book air 6 months ago, I am using the cable and power box that came with it and haven't had any problems with it for the past 6 months.

Last week I realised that instead of the power percentage going up while on charge, it was slowly going down. I have been charging at the same power outlet for the last seven months so that shouldn't have anything to do with it.

I've tried to restart my laptop but no luck, is it a problem inside the actual computer?

Do I need to take it to a tech shop so they can open it up and look for the problem?

Or is it something in settings that I am missing?

Thanks :)