Is "My bike was stolen" correct?

Why

Ayush has correctly noticed that many textbook passive examples include a "by" phrase — "the cake was baked BY her" — so it's reasonable to think that piece is a required part of every passive sentence. One of the passive voice's most useful jobs, though, is letting a speaker leave the doer OUT entirely, exactly when the doer is unknown, unimportant, or obvious from context — "My bike was stolen" works perfectly without saying who stole it, because the speaker likely doesn't know who did it, and the sentence's real point is what happened to the bike, not who's responsible. Forcing in "by someone" adds nothing true or useful; "someone" is such a vague filler that leaving it out entirely is the more natural, correct choice. Test: compare official announcements like "The rules have been changed" or "Mistakes were made" — these are completely standard, correct passive sentences that deliberately omit who did the changing or the mistaking. No, it is correct without "by": the passive voice can drop the doer entirely when it's unknown or unimportant, which is one of the main reasons the passive voice exists at all.