My home page got black listed suddenly

Hi, It looks like the home page of my client got black listed suddenly. Only the results of keywords associated with the homepage got slapped to page 8 or that the keyword is not associated anymore to the homepage, but to inner page. Rest of the results - that were associated with inner pages didn`t get slapped. It happened yesterday.. Is there any way to investigate the reason for that? It might be that the previous SEO company sabotaged the homepage? Is there any way to prove it? Thank you!