I recently had this exact problem after my Alexa app on my iPhone was updated. The reason the Echo Dot stopped responding to "Alexa" was because the Wake word was changed to "Amazon". In other words, I would have to say "Amazon, what's the weather?" for the Echo to wake up, instead of asking "Alexa, what's the weather?".
To change the Wake word back to "Alexa" open the Amazon Alexa (iPhone) app or Android app -> then go to your device (i.e. Echo Dot) -> then scroll down to the "Wake Word" -> Your wake word is what you say to get Alexa's attention. Changes will only be applied to this device. Currently, you can change the wake word to either: Alexa, Amazon, Echo, or Computer.