Assuming that the upgrade did go without errors, you might need to remove the licence key from the database.
You could run a query to remove it, and see if that resolves the "yellow screen" issue. You can then add the licence through the Admin area, as Brenden described.
Here's a query to list out all the licences (use SQL Server Management Studio to execute it):
SELECT * FROM CMS_LicenseKey
Run a query with the
DELETE statement to remove the licence (or licences), and then restart the application pool in IIS.
DELETE FROM CMS_LicenseKey WHERE LicenseDomain = 'yourdomain.com'