najednom

1. suddenly; all of a sudden
   Najednom je nestalo struje u celoj zgradi. — Suddenly the power went out in the whole building.
   Pričali smo mirno, a on se najednom naljutio. — We were talking calmly, and he suddenly got angry.
2. all at once; simultaneously
   Ne mogu sve najednom. — I can't do everything at once.

In everyday speech odjednom is more common.