all at once

(idiomatic) Unexpectedly; without warning; all of a sudden.

Example: William Wordsworth, Daffodils
  When all at once I saw a crowd, / A host, of golden daffodils
1913, Joseph C. Lincoln, chapter 4, Mr. Pratt's Patients:
  I was on my way to the door, but all at once, through the fog in my head, I began to sight one reef that I hadn't paid any attention to afore.
Used other than as an idiom: All at the same time; all together.
  There are too many to get in the lift all at once, so some must wait.‎