Is it possible to do graduate school with low marks? (i.e 60s, 70s)
Would the only way of getting into grad school be to do another degree and start anew?

60's, probably not. Some programs have cutoffs in the 70's.
Experience tends to carry more weight than marks for grad school (in science, at least).

I don't know about 60s, but if you are in engineering you can get into some graduate programs with low 70s.

All depends on the program's requirements. If it's a non-competitive program...then likely they'll accept you.

If its a competitive program, then you'll need something else to prove that your better than the other applicants (i.e. good work experience, volunteer work, etc.)

You better have a shit ton of research, reference letters, publications, and papers under your belt.