a=0
or
b=0
(c any rational number <>0)
by this you will only get a point but no line
if a=b=c=0
even this will not represent a st. line
ax+by+c=0
then
a=-(by+c)/x
now if y,c,b,x are rational numbers then a will also be a rational number (case of 0 has already been discussed)
so i don't think there is any value of a,b,c (except a=b=c=0) for which equation ax+by+c=0 do not represents a line.