Urgent-Sharepoint default.master "File Not Found" error - Julian

I was trying to test sharepoint designer, and I tried testing just adding a
button to the defaul.master page, and saved it. I then tried to open the
sharepoint site up and it gave a "File Not Found" error. We then took the
button out a made the default.master how it was before, and it still is
giving us the same error. Can someone please help us with this. Is there
anyway to reset the default.master or get it working again?

Thank You