Amazon provides freely a basic instance monitoring, which includes a System Status Check, an Instance Status Check and a graph generated for each hour of instance activity.
In the Instances section, click on an active instance and expand the bottom window for the Status Check panel.
System Status Check monitors the AWS systems involved with the current instance and ensures they are functioning properly, while the Instance Status Check monitors the network reachability of the instance (with ping).
When both checks are returned as success, then you should be able to reach the instance without any problem.
In the Monitoring section, you can find a bunch of statistics graphs, generated every 60 minutes, which can be used to monitor CPU, disk, and network usage.
If you enable the additional CloudWatch service, you can use the Enable Detailed Monitoring button to get more interactive graphs that get updated every minute, and the ability to receive e-mails when predefined...