I was doing a vc-relax calculation and after 1 day the calculation suddenly stopped. Is there a way to restart it from previous interrupted calculation of do I have to restart from the beginning?
You need to have prepared for this beforehand. You need to write out the wavefunctions in a place that is accessible for the new run to absorb in, and so forth.
But if you still have the interrupted output file, you can read off the cell parameters and the atomic positions just before it crashed, and start from there. Even if you did not have the wavefunctions to restart from, it will still be vastly faster.