WordPress Header reverted to default

I recently logged onto my website to discover that the header had reverted back to the default header that came with the Twenty Ten 1.2 theme. I had originally changed it to my own image made to the same size as the default and I uploaded it on the wordpress dashboard as a ‘custom header.’ It worked for a couple of months, but now it has reverted to the default and I can’t seem to change it back.

My website is https://www.cdhhp.co.uk. Does anyone have any ideas as to how I can change it back to my custom header?

Thanks