Once one and then you might experience that your VM hangs with the status “Starting”.
This is really frustrating as you do not have a force power off button in the Azure GUI.
This is where PowerShell is your savior.
Open a Windows Azure PowerShell window, and connect to your subscription. The run this command
Stop-AzureVM -Name YourVMName -ServiceName YourServiceName -Force
Force is the keyword here. Give it some seconds, and the VM should stop. Now, you can try to start it again.
For more on this command, and Azure Powershell in general, visit http://msdn.microsoft.com/en-us/library/jj152831.aspx