How to fix a stuck task
It is possible a task may run longer than expected and you suspect something has gone wrong. It is even possible the task may cause a server to become unresponsive. Restarting the server will not stop the task, although it may make the server become responsive again.
To stop a Task, find the related task node (install: mysite.com) in the content list: /admin/content/node/overview, and delete it. This will stop Aegir from trying to run the task.
In the case of a site install, it may also be necessary to also delete the node for the site itself. The site may have actually installed properly. Verify the affected platform and see if Aegir imports the site. If so, you may need to reset the password to gain access to the new site. If the site doesn't import in the verify, delete any files from the sites folder on the selected platform and either try again and/or research why the task may have failed.