Hello,
It depends.
1) How old is your website?
2) Was it good ranking for a long time and suddenly fell?
3) Are your ranking competitors stronger than you?
4) What kind of off-page techniques do you use?
Any much more.
Possible answers to ranking fail:
-Page entered in "google dance" mode - google will do down and up rankings to test if site fills your google position
-Google devaluated your site because of some possible blackhat SEO / Google update downgraded your ranking because
-Your competitors made better off-page SEO, and you fell
Wish this helped you, but there are much more details around SEO!
Best,
Damie