Distributed systems can make it difficult to debug problems, and you need to plan for this in advance by integrating technology that can help with visibility. You should know what metrics you want to measure and what parameters are important to record.
As you run a web application, it isn't a case of deploy and forget, as it might be with mobile apps or desktop software. You will need to keep a constant automated eye on your application to ensure that it is always available. If you monitor the correct performance metrics, then you can get early warning signs of problems and can take preventative action. If you only measure the uptime or responsiveness, then the first that you may know of a problem is an outage notification, probably at an unsociable hour.
You may outsource your infrastructure to a cloud-hosting company so that you don't have to worry about hardware or platform failures. However, this doesn't completely absolve you of responsibility, and your software...