いきなり (ikinari) means “suddenly” | MotiTan Japanese Dictionary

More examples

電車がいきなり止まりました。den sha ga i ki na ri to ma ri ma shi ta

The train suddenly stopped.

Presents an event or action as arriving abruptly, with no expected lead-in or preparation.

突然totsuzen: Something happens unexpectedly and without advance notice.
急にkyūni: A change or event takes place quickly and unexpectedly.

前触れや準備がなく、物事が突然起こるさま。mae bu re ya jun bi ga na ku mono goto ga totsu zen o ko ru sa ma

The adverb いきなり (ikinari) is commonly analyzed as 行き成り (ikinari), combining 行き (iki, "going") with 成り (nari, "becoming").

In everyday conversation, いきなり (ikinari) often describes someone who starts speaking or acting without a conversational lead-in, not only an event that happens suddenly.