Help on the back end of my site

I have a wordpress site but it has developed an issue. when I go in to edit it, the format is completely different from what I understand a standard the WordPress edit format should look like and it doesn't let me edit anything. I have a decent ranking on Google so I can't reinstall the template or anything like that as i will lose the ranking as I'm aware. The site works fine I just can't edit anything on - is this something you can do?