trouble with photoshop

Im using adope photoshop, i dunno which version, probably 7, but anyways.. it was working fine yesturday and all the days i used it, and now its not working. I dont know what happend. I cant put layers on pictures, and when i do paint, i cant make it ligher , its only dark, and when i do use the brushes, the modes are either threshhold or dissovle, and thats it. Does anyone know the problem and how to fix it?

thankyou to anyone that helps!!