A sudden drop in Google rankings within one night ?

I don't know why my keywords rank drop from page 1 to page 3 in google search results ?
Some keywords for website:
diendanthammy.com – for example: "mat na tri mun" from the position 5 on page 1 to page 3, "thuoc tri mun" from the position 7 page 1 to page 6 and "tri mun dau den" from page 1 to page 2.
what should i do now?

See original post by Mrchung