Text box not appearing in a Messages conversation
From another site, try disabling and then re-enabling the account in question as follows. 

Within the Messages app:


Select the conversation that has the problem (I don't know that this is necessary, but I did it anyway)
Go to Messages->Preferences...->Accounts
In the box at the left hand side, select the account in question -- it will probably be your Apple ID with "iMessage" below it
Then over on the right, make sure "Settings" (and not "Blocked") is selected near the top
Still on the right, underneath your Apple ID, uncheck "Enable this account"
At that point, back in the main Messages app you should see the text box reappear at the bottom of the conversation
Then re-check "Enable this account"


Done.

P.S. This was done on OS X El Capitan (Version 10.11.1), and Messages app version 9.0 (5084)