Ok so i've been working with layer masks for the last few days, yet for some reason today they wont work like they have been. I'm wondering whether i've accidentally hit/clicked something i shouldn't have.

For eg. I used to be able to copy the background, add a layer mask.
I've been using filters occasionally and when i use a layer, i can't use any of the filters. When i select it, it comes up with a blank grey or White screen.

Does anyone have any suggestions as to why this is happening?

Thanks in advance to any replies