If you use the remote, you don't need to say Alexa but you do need to hold the button down while you give the command and let go as soon as you've completed the command. Don't keep your finger on it (as I did at first) waiting for it to respond because it won't.
If you don't use the remote, you do need to say Alexa.
I use the remote when there's a lot of noise around or when I have the Echo's volume turned up high because it's easier to make voice contact. Also when I've set the morning alarm on the Echo in the kitchen and want to turn it off from the bedroom. The rest of the time I just use voice directly. I don't know why yours worked once and not since though. Do you mean you can't get it to respond at all, whether via the remote or directly?
Another possibility I've just thought of (and most likely isn't relevant but here goes), you haven't somehow changed the wake word have you? There are three: Alexa, Echo, and Computer. You can check in the app which is assigned to your device and you can change it to one of the others if you prefer. My Echo is called Echo and the two Dots are called Alexa, but every so often I use the wrong one and wonder why it's not responding.