Phil,
I have had this happen after updates, especially if you are using a firewall.
Try starting up in safe mode with networking (F8 on boot). Then shut down and restart normally. This is usually enough to get you going again.
You may also be in a screen resolution not supported by your monitor, try logging in as administrator (a different profile).
Your graphics card driver may be sideways, after you log in is when it takes affect. The windows splash screen is low res VGA mode.
-Fred