I'm running the linux distro on my server.When i want to verify the size of the disk, i'm issuing this commnand to get the output.

 df -h

But it does not produce ANY output.Strangely enough when i'm issuing other command such as fdisk -l or du -h it can show output normally.

Does anyone now why is this happening?Thanks.

edit:

here is the output of cat /etc/fstab

 none     /dev/pts       devpts   rw   0     0

and this is for mount command

 none on /dev/pts type devpts (rw)
 none on /proc/sys/fs/binfmt_misc tpe binfmt_misc  (rw)

edit(2):

here is the output of cat /proc/mounts

 /dev/vzfs / vzfs rw,relatime,usrquota,grpquota 0 0
 proc /proc proc rw,relatime 0 0
 sysfs /sys sysfs rw,relatime 0 0
 none /dev/tmpfs rw,relatime 0 0
 none /dev/pts devpts rw,relatime 0 0
 none /proc/sys/fs/binfmt_misc binfmt_msc rw,relatime 0 0