got a problem with photoshop, wheneva i open a photoshop file (with the layers etc that i've been workin on before and go to use the eraser tool it doesnt work, but if i add anything i can eraser it but then if i save it and exit photoshop then reopen it the eraser tool doesnt work

its neva done this before and i'm really stuck, i have just instealled norton firewall 04 on the system but i've used it before and neva had this problem before

anyone help please