You really do need to provide more information, but if you don't have it, and if you're in Linux, start with some of the below. Note that different Unixen have very different tape device (/dev/*) naming conventions, and that the leading letters often specify whether the tape autorewinds after a use, uses/expects compression, etc.
dd if=/dev/rst0 bs=1024 of=data # use the right /dev/* for your drive
# and try 512 instead of 1024
ls -l data # look for the output being at least bs=<whatever> bytes long
file data # this may produce info on what format data is on the tape
less data # also may provide some idea of the data format
or:
tar tvf /dev/rst0 # same /dev/* note as above
These may provide useful info, but only if you're lucky, there are a lot of different formats the data could be in, and many ways said data could be written to the tape.