OK, here is exactly what has happened.
Last week, the site had two brute force attacks. Our server shut down as a security measure (as it should) each time and was offline for a couple of hours before being getting back online.
A post attack conversation with the company who look after our sever led to some new software being installed on the machine to protect it from such an attack again.
This has caused the blank post problem as there is a clash with Kunena, the forum software. We tried upgrading to the latest version of Kunena, but that caused problems with the CMS.
So the bottom line is we will have to do a huge upgrade to not only the CMS, but every module and extension too. It's not a job that can be done in a day as it has to be tested on a copy of the site before being deployed here.
As soon as we can it will be fixed. Copying the text before you post is always a good idea.