I saw a post identical to my problem on the Help Board, that wasn't resolved. Thank you for posting-- I have found a solution! (Added to bottom.)
"When I first purchased an Amazon Echo (Alexa), I was able to easily link it to my Spotify premium account. I could say things like “Alexa, go to Jon’s Spotify and play Blueberry Hill by Fats Domino” and bingo, it would work. Then all of a sudden, it no longer connects to Spotify. Instead, Alexa tells me that I have to link the Alexa app to my Spotify premium account – which I have already done. I have also designated Spotify as my default music service, to no avail. I’ve tried everything and consulted countless message boards to find a solution but I can’t get Alexa to connect to Spotify, as it once did. Yes, I can add the Echo to my list of Spotify devices on my iPhone and iPad but that arrangement does not allow me to use Alexa as a voice controller. I can also connect the Echo using bluetooth but that likewise eliminates voice control. (If I was simply looking for a remote speaker, I would connect Spotify to my Sonos system which gives way better sound than the Alexa speaker, and in stereo with a subwoofer to boot). I’m ready to get rid of Alexa and replace it with something like Google Home but I’m willing to try something else if anyone has any ideas. Thanks for any suggestions you may have! "
Yes-- I am not insane our alone. That is my problem. Here's what worked for me-- having spent over an hour searching around and trying things. Opened my Alexa app on my PC, selected my Echo, went to "Settings", scrolled down to "About" and clicked on the "Deregister" for my Echo. Then I re-registered it, and it works-- I can use the voice-control. Finally.