It depends on how you cancelled, stopped or paused the execution, some actions will not allow you to resume.
Please see this post: https://jazz.net/forum/questions/94303/how-to-resume-the-test-case-execution-if-we-cancelstop-run-in-middle-of-execution

The referenced workitem has more explanations:
https://jazz.net/jazz02/web/projects/Rational%20Quality%20Manager#action=com.ibm.team.workitem.viewWorkItem&id=62649