If this is a real time tournament, did you check in? For real time tournaments, you need to check in before the tournament starts. The check in window opens 15 minutes before the tournament starts and then closes at the official start time. If you do not check in, then you will not be participating in the tournament and will not be eligible to win any prizes.
If this is an any time tournament, did you meet the SPS stake requirements and the collection power (CP) requirements? For Splinterlands tournaments, you need to meet the SPS stake and collection power requirements both when you sign up for the tournament AND when the tournament begins. If you do not meet the requirements when the tournament begins, then you will not be participating in the tournament and will not be eligible to win any prizes, though the entry fee should automatically be refunded to you. If you rent cards and your collection power total is right around the threshold of the minimum requirements, then it is advisable that you double-check to make sure your rentals do not elapse before the tournament otherwise your collection power might drop below the required level.