DIAGNOSIS The skill linkage between your MyQ account and Alexa has probably expired or lost its authentication token. It's a common cloud-service handshake problem.
FIX You need to disable and re-enable the skill. 1) Open your Alexa app. Go to Skills & Games > Your Skills. Find the "MyQ" skill and disable it. 2) Go to your Chamberlain MyQ app. In the menu, go to Integrations and make sure the Alexa tile says "Disconnected" or similar. 3) Now, in the Alexa app, search for and re-enable the MyQ skill. Go through the login process again, using the *exact same* MyQ account credentials you use in your Chamberlain app.
IF THAT DIDN'T WORK Check the names of your devices in the MyQ app. Alexa can get confused by special characters or very long names. Try renaming your opener to something simple like "Garage" in the MyQ app, then rediscover devices in the Alexa app.
HEADS UP The MyQ skill sometimes has regional issues. Make sure your MyQ account was created for the correct country (US vs Canada, etc.). Using a VPN on your phone can also break the skill linkage.