Likely it is because the text box control is set to protected. Either set the textbox control to public or the better approach is to add a property to the previous page. Something like below. Note I would not name like I did a page PreviousPage. It is for example only.
You can go though following link to get it work
Or Instead of this you can simply use ViewState or Session to store textbox value also.
Mark as solution if its solved your problem