now Airport Express is not recongized
This would be expected since the Express still has the stored settings to connect to your "old" wireless network....which now no longer exists. The Express is still trying to connect to your old network, and it cannot do so.
You need to start over on the Expess and reconfigure it to extend the "new" network.
The Roku as well will need to be reconfigured to connect to your "new" network.