Should I throw away the soil from my box garden and get new dirt ??

That is the usual advice. If there was no disease in the previous plants and you can add compost to replace nutrients you can continue to use the same soil. In my opinion.

If the soil was good to start with, then no. Just add some slow release organic fertilizer and plant when your weather permits!

Depends on how the soil feels and looks. If hard or looks white on top, no. Otherwise add some new with fertilizer and good growing!

I always put in a little new potting soil to replenish what nutrition was used up and the make sure to use fertilizer to help replenish it. Make sure you break up the soil you have in the garden and mix in the new with it, or put it in when you plant. Sometimes I do it as I plant by making the hole deeper and wider and using new potting soil to fill it in. That way you know the roots will have a good start.

If anything planted in the dirt got sick, died or was attacked by a mold or such, then toss the dirt and start fresh. Otherwise, add about half new potting soil and don't forget to fertilize, either the kind you work into the soil now or add after planting.