site fell off the charts?

First I am a beginner and not a professional web builder/SEO expert. I built my site myself hand wrote it using a text editor and I had optimized it for SEO, For many years it has appeared on page one of the organic searches even after I have re-written the site. Now with no major changes to the site it, the home page does not appear anywhere in the specific keyword search even fifty pages deep. I could understand a drop to page three, four, ten, etc.. But for a site that was always come up on page one to not appear even for fifty pages I am hoping someone can suggest some ideas for me to look at?

Any insights would be appreciated and thank you for your patience with a beginner. Jeff

p.s. in case you want to look here is the site http://www.jeffwolfram.com