undo css editing

Hi,
I tried to change the layout so that the search bar disappears. In order to do so, I followed the changes suggested by some guy on the internet.

But, my editing changed the layout and is unusable now… and I couldn’t even undo it by deleting the changes. How come?

And is there a way how I can either undo the change?
Or – is there a way how I can reset the code to the original code?

Thanks for your answer!

Ps. my site is akertconsulting.ca