Text box not appearing in a Messages conversation

Solution 1:

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)