Why ID don't let me save or undo?

On the menu "File", the options SAVE, SAVE AS, REGISTER and SAVE A COPY are disabled. And on the menu "Edit" the options UNDO and REDO are also disabled.

The file says on the bottom: no errors.

It had happened to me twice, but first time I didn't care on it, because I thought it was a punctual error; but working on another document it happened to me again. There are different files, I can't solve this.

I have tried to copy and paste on another document but it doesn't let me save again.

I have a MacBook Pro and ID CS6. I need to save the document. What can I do?

Thanks!