The first step is checking the Estimated Delivery Date on the project page. Backing a project is a lot different than simply ordering a product online, and sometimes projects are in very early stages when they are funded.
If the Estimated Delivery Date has passed, check for project updates that may explain what happened. Sometimes project owners hit unexpected roadblocks, or simply underestimate how much work it takes to complete a project. Project owners are expected to communicate these setbacks when they happen.
If the project owner hasn’t posted any update, send them a direct message to request more information about their progress, or post a public comment on their project asking for a status update.