Cannot start service SPUserCodeV4 on computer

I encountered this error when I was trying to deploy a solutions right from within SharePoint 2010. It seems this is because a necessary service is not running. The particular service we are talking about here is called "Microsoft SharePoint Foundation Sandboxed Code Service".

To turn it back on:
* Go to Central Administration -> System settings -> Manage services on server
* Locate the service and turn it back on

This should allow you to again deploy solutions from within Visual Studio 2010.