Help needed with seo and why Google has dropped my main keyword

I have a website which usually appears on page 1 of Google, usually about position 5. A couple of days ago it dropped from page 1 completely and just doesn't seem to rank at all.

I need someone that can identify why as it is very important. I do not want someone to tell me why it might because I can Google that. I need to know exactly why and how I can fix it.

I don't think Google has penalised me as I still appear for other keywords.

If the right person comes along then I also need some link building to be done and also my website optimised etc, etc. All the usual seo stuff because I have done everything myself so far and thought I had done a pretty good job.