- Did you by any odd chance change your wifi network information or password recently? Devices (and any related hubs) need to be connected to the same wifi network as your Echo in order to control them with Alexa
- Did you have any issues with a slow internet connection on the days when the Alexa app stopped recognizing the devices? Sometimes these issues solve themselves
- If you are using a Smartthings hub, have you tried resetting it and then going through the set-up with the devices again with the Alexa app? Here is a link for how to do it: https://support.smartthings.com/hc/en-us/articles/204936890-How-do-I-factory-reset-the-Hub-delete-a-Location-