I had a similar problem. My Roku Express 3910 was on version 9.3 and wouldn't update to 10.0. Pretty soon DisneyPlus stopped working, and I presumed other channels would follow suit. When I did the manual Check for update via the System menu it listed 10.0, and the update appeared to succeed. It said "Restarting", and then seemed trying to restart, but then the screen went blank and I got the "No signal" message. If I unplugged the Roku and plugged it back in, it once again listed version 9.3.
I tried this multiple times and the same thing happened. (I didn't restart our router, but the Roku said its last update was 9 months ago and the router had been restarted many times since then ... it apparently checks for updates every day, so I figured they were failing every day.) I moved the Roku to another, older TV and it had the same issue.
Here's how I fixed it:
1. Selected System > Advanced Settings > Factory Reset. (Note - this removes all your channels, so you'll have to add them back again).
2. Re-found my wireless.
3. Roku immediately popped up an "Updating software" dialog and did the update.
4. Once again when it restarted it wouldn't come back (No Signal).
5. When I unplugged it and plugged it back in, it once again gave the Software upgrade dialog and updated the software.
6. This time, for whatever reason, the restart worked. (Huzzah!)
7. I entered my account name (email) and re-activated the Roku by clicking on the email it sent. Viola, it was on version 10.
8. I added back all our channels. DisneyPlus is now happy!