Place your frozen grapes in a bucket in an area that will stay consistently around 70 degrees Fahrenheit overnight.
Stir the grapes as they thaw, removing those that are freezer burned as well as any stems.
Pour the thawed grapes into a large container and mash with a potato masher until juicy. If you're using the juice for wine, strain with a cheesecloth and proceed with your wine recipe.
Dump the mashed grapes into a large cooking pot to make non-alcoholic juice. Heat over medium until the mash just starts to boil, then reduce the heat to low and simmer for 10 minutes, stirring occasionally.
Place a large, fine mesh sieve over an empty pot. Ladle the cooked mash into the sieve and press it against the mesh to squeeze the juice out. Empty the sieve as necessary until all of the mash has been strained.
Rinse the sieve and place a coffee filter inside. Place the filtered sieve over another clean pot or bowl and pour the strained juice through the sieve one more time.
Transfer the juice into a pitcher or bottles and refrigerate. The juice can be consumed as soon as it's cool.