It depends. If you have opted for debt-based crowdfunding or peer-to-peer lending, then yes, you will have to pay back the money. Some sites also subscribe to an all-or-nothing system, which means you will forfeit all the funds you have raised unless you meet the goals you've set. Carefully read the terms and conditions of the platform you've registered with, and make sure you tell potential funders whether or not they can expect to be repaid the money they have invested in your project.