cat shows nothing

Your file is full of nulls, rather than empty. A regular cat will print the nulls to standard output, but your terminal will generally display them each as nothing, while cat -v represents them as ^@. ^@ represents a null byte because the byte value of "@" (0x40, or 64) xor 64 (flip bit 7) is zero.

Why it's suddenly full of nulls, we can't tell from here.

This related question may be informative about the caret representation: Are ASCII escape sequences and control characters pairings part of a standard?

Be careful , cat is not the perfect tool to display data that can be binary.

A simple unix tool is od ( octal dump ).

A example of od -c -tx1

root@server:~# od -c -tx1 /etc/passwd | head
0000000   r   o   o   t   :   x   :   0   :   0   :   r   o   o   t   :
         72  6f  6f  74  3a  78  3a  30  3a  30  3a  72  6f  6f  74  3a
0000020   /   r   o   o   t   :   /   b   i   n   /   b   a   s   h  \n
         2f  72  6f  6f  74  3a  2f  62  69  6e  2f  62  61  73  68  0a
0000040   d   a   e   m   o   n   :   x   :   1   :   1   :   d   a   e
         64  61  65  6d  6f  6e  3a  78  3a  31  3a  31  3a  64  61  65
0000060   m   o   n   :   /   u   s   r   /   s   b   i   n   :   /   u
         6d  6f  6e  3a  2f  75  73  72  2f  73  62  69  6e  3a  2f  75
0000100   s   r   /   s   b   i   n   /   n   o   l   o   g   i   n  \n
         73  72  2f  73  62  69  6e  2f  6e  6f  6c  6f  67  69  6e  0a

so you can see that the carriage return \n