iMovie sudden import HD problem

Using camera Sony Cybershot DSC-HX20v to record HD video. According to manual if using a Mac I have to use existing system software ie iMovie.

So I put in my SD card, click camera import window button in iMovie and the videos Ive recorded are there - tick the ones to import etc.

I took 2 videos yesterday - went through this process and it worked fine. Later i took some more videos, put in my SD card again - now when I click the camera import button all that flashes up is the iSight onboard camera and starts filming my face! No option for an external camera/memory card etc. It just started doing this with no warning doesnt seem to know the card (or camera if I plug in USB) is there - wont show videos new or old. Been like this for days. What the **** is going on?

One minute seems fine, next completely the opposite.......