There seems to be two issues, before we get to the issue of reconnecting the wifi you will first need to address the "No Signal" that you are seeing on the TV.
When they connected the new cable box, they may have unplugged the Roku Express or moved it to a different HDMI input on the TV. Try to trace the HDMI cable coming out of the back of the Roku Express and find which input it plugs into the TV. Also make sure the Roku Express is plugged into power.
Then find the TV remote and press the Input or Source button until it gets to the Roku home screen.
Once you are able to get to the home screen, you can move on to connecting to the new wifi.
Key thing to know: you do *NOT* need the old password. That only existed for the old wifi and the old wifi is gone. Neither the Roku or new wifi will require knowing the old password at this point.
You *WILL* need to know the Wifi network name (frequently listed as "SSID") and the *new* wifi password.
On the Roku:
- Press Home
- Scroll and select Settings
- Select Network
- Select from the list of available wifi network names
- If the wifi network name is missing, select Scan Again
- Once you selected the network, then use the Roku remote to "type" in the wifi password
- Highlight Connect and press OK
This video from Roku's official YouTube channel might be helpful if you find the step by step instructions confusing:
https://www.youtube.com/watch?v=OoocXCd1ti4&t=39s
Good luck!
