Nothing showing in the error log. Please describe what took place, exactly. What happened when you clicked on the submit button? Did you get a message that said "The message has been submitted successfully."? Something else?
Although not the answer you were looking for, once in a while things happen like automatic server reboots and cache dumps that can affect contents of an open post window. The habit I encourage everyone to get in, is to either type long posts in a Word or text document then cut and paste, or, after typing everything in the post window, highlight and copy what you typed prior to hitting the submit button. The latter of the two suggestions is what I personally prefer.