I second J English Smith's comment - the CPU card must be pressed VERY firmly to engage properly. It makes a click/pop noise that you can feel when it has engaged. You have to do this with your thumb for adequate pressure. Once engaged, it is very difficult to disengage, often breaking the plastic tab that is meant for removing the CPU card.

It could be that you had the CPU card mostly engaged and then it popped itself out a bit as you were using it.