While some style guides might suggest avoiding starting sentences with adverbs like "suddenly", it's not grammatically incorrect. In fact, starting a sentence with "suddenly" can be effective for adding emphasis or creating a sense of surprise.
Examples:
* Suddenly, the lights went out.
* Suddenly, a loud crash echoed through the house.
* Suddenly, I realized I had left my keys at home.
However, it's important to use "suddenly" judiciously. Overusing it can make your writing seem repetitive or lackluster.