The correct answer is 'Is the power outlet functioning?' because it's logical to check if the power source itself is working before investigating more complex issues with the computer's internal hardware or software. If there's no power being supplied to the computer, it won't turn on, no matter if everything else is in perfect order. 'Are there any error messages?' would not be applicable since the computer is not turning on at all. 'Have you tried using another device?' can help determine if the issue lies with the computer or other components, but it's not the most direct next step. 'Did you install new software?' is unrelated to a power issue, which is what is being reported in this scenario.