I have exactly the same problem.
On both my Roku Ultra and Roku 4, the Spotify channel disappeared from the "Home" screens, but was still showing as being installed (under Recommended).
When I saw it listed under "Recommended", it was showing as still being installed... I selected it from that screen, and went to "Got to Channel" it launched, but did nothing, other than cause it to reappear on my home screen (at the end of my channel list).
"DownDetector" showed that Spotify users were reporting a LOT of issues yesterday evening. I hope that it resolves itself quickly.