For some reason that, I imagine, involves the transition to unified Google+ instant messaging interface, Gmail had recently introduced a 'new' composer functionality, which looks like this and is, frankly, absolutely unusable:
While switching to it was as easy as clicking a friendly blue button that magically came up one morning, switching back proved to be slightly more challenging, so I figured I'd post it here for anybody who may also be looking for a way to switch the new Gmail composer back to the old one:
Hopefully Google reconsiders making this the default/only option available for the Gmail composer, because it is very hard to use and wastes valuable screen real estate. However, if you do prefer it and want to turn it on again after disabling it as per above screenshot - it is very easily done at the compose screen:
I hope this helps!