Get-AzureVM Returns Nothing
Run Add-AzureAccount; this will load the Azure login dialogue, allowing you to sign in.
Once done, running Get-AzureSubscription should work as you'd expect.
Sadly no helpful error's displayed to say "you're not logged in; run Add-AzureAccount to login" if the command's run without you being logged in.
If you are attempting to fetch ARM resources, please use “Get-AzureRmVM” cmdlet.
Get-AzureVM - To fetch classic instances
Get-AzureRmVM - To fetch ARM instances.