The status checks automatically performed on Amazon EC2 instances are:
See documentation: Status Checks for Your Instances
Often, an instance is available and ready to be used before these checks are complete -- this is especially the case for Linux instances because they boot very quickly.
If you receive a
1/2 checks passed message, either wait a little longer or Stop and Start the instance. Performing a Stop/Start will launch the instance on a different host, which will probably fix whatever problem was being experienced.
1/2 checks passed message continues to appear after a Stop/Start, it is probably a misconfiguration of the AMI. I have seen this when the wrong virtualization type was selected for an AMI that was created from a Snapshot.
You might be able to get a hint about the problem by using the Get System Log command in the Actions menu, which shows the log while the instance is booting.
Worst case, launch a new instance from a known-good AMI, attach the non-booting volume as an additional disk and copy files to the new disk. You will still have access to your files even if it will not boot.