AWS CodePipeline frozen on Source
There should be a log group available in CloudWatch that you can refer to for information on the error. Also, there is an Execution History tab available on the left hand side that will let you see any errors that are not logged to CloudWatch. 

From the sounds of it there may be a problem with the source repo you have selected. It should only take a few seconds to read the repo. Very generally with AWS, many of the problems can be traced to IAM permissions.

You can cancel the execution by selecting 'edit' while it is running.