hi sama, try Unlocker- if its the usual error of 'cannot do bla, it is being used by another person or program' type error http://ccollomb.free.fr/unlocker/
I also had this problem recently. I do not know why it does it, but I found it would give the error if I tried to render a file format which was not supported by mental ray. The renderer would switch itself to maya software once it failed to render the .psd fail and from that point on, I am unable to save the file.