There are some cases where distance time graph of a particle is vertical . so could you tell me when it is

-- If time is the horizontal axis and distance is the vertical axis, then it's not possible
for the particle's distance/time graph to be a vertical line.  That would indicate that
the particle covered some distance in zero time, indicating infinite speed.

-- If distance is the horizontal axis and time is the vertical axis, and the particle's
distance/time graph is a vertical line, then that indicates that the particle's distance
doesn't change with time. In other words, the particle is at rest, and its speed is zero.

Did you ever figue this