Is Espruino in 'bootloader mode'? The LEDs on it will be glowing on and off if it is.
Do you have another device plugged in that you might be connecting to instead?
Might you have reset/unplugged Espruino without clicking the disconnect button in the Web IDE? If so you'll have to make sure you completely exit and restart Chrome. It's a longstanding bug in Chrome that they've only just fixed - but it won't make it into normal builds for another month or so.
Espruino is a JavaScript interpreter for low-power Microcontrollers. This site is both a support community for Espruino and a place to share what you are working on.
Has it just started doing this for you?