Why are my text layers changing? Its stopping me from editing the text..

I created text boxes with with the type tool and put text in them. When I came back to them later to resize the text, they wouldn't let me edit them. It looks as though my text layer has changed?

Does anyone know why has this happened and is there a way I can change them back to let me edit them?

I have PS CS5. It looks as though the layer has rasterised but at no point did I rasterise it and it's happened more than a few times with this work. I've had to go back and recreate all of my text boxes which has taken me a while.

I would like to try and find out why it's done this so I know if it happens again. I've never had anything like this before using PS so I'm a bit puzzled! There was no error messages and I've checked the fonts and they're fine. Sorry if it's a silly question but is there something that will rasterise text layers without you actually selecting 'Rasterise Type' from the options?