You have been trying to start up the User Profile Synchronization Service and it has been in a starting state for hours/days/weeks.
Run following commands in the SharePoint 2010 Management Shell to stop the service:
get-spserviceinstance > driveletter:outputfilelocation.txt
Find out the service ID for user profile synchronization service and then use this command to stop the service:
I have used this same method 3 times in the past week to stop different SharePoint 2010 services for various reasons. Worked like a charm every time.