Our logo has gone missing from the header of our site!

Hi there,

I've just uploaded a new page to our website, and when I returned to the site, I noticed that the logo which appears in the header of every page of our site has gone missing! There is now just 'holding text' with our company name in its place!

I'm sure it's a really simple job, but can someone just tell me how I go about re-instating our logo in the header? Screenshot attached so that you can see what we've currently got.

Thanks so much for your help.