When using an iframe, you will first have to switch to the iframe, before selecting the elements of that iframe
You can do it using:
driver.switchTo().frame(driver.findElement(By.id("frameId")));
//do your stuff
driver.switchTo().defaultContent();
In case if your frameId is dynamic, and you only have one iframe, you can use something like:
driver.switchTo().frame(driver.findElement(By.tagName("iframe")));