Hi
Without seeing the site or search term(s) involved... all you have done is invited speculation as to problems.... and in return you will receive a ton of misinformation that will have you chasing your tail, making changes that will hurt the site further, and in general making things more confused than they are.
The best advice is to sit tight and see what happens.
I had a possible client recently who had a site established in the # 1 position ( I understand yours was first page) for years whose site had recently disappeared.
He wanted me to fix things with
SEO and to start immediately.... I had to turn him down as I knew within a week or two things would likely be back to normal.
I advised the gentleman to wait a couple weeks and then if nothing had changed, to then contact me.
Needless to say he did not need to contact me....
Be patient..Google takes months to score sites and is never in a rush to react..