This is a long shot, but go check that your Nvidia driver's startup type under "services.msc" is set to "Automatic".
The same thing happened to me with a beta driver once . It turned out that the startup type for the new driver installation was set to "Disabled". The startup type for your driver should be on "Automatic" by default.
Note that you'll need to restart your PC after changing the startup type, if ever.
Did you do a clean install of the drivers? Is your 550 Ti a recent upgrade?