This is simple - just visit the grocery list page and resync the grocery list for the rest of the current week. Then start following the meal plans as soon as you can.
If you miss your grocery shopping day, you might have also missed a few days of the meal plan. Remember to delete stuff you don't eat and add what you do so that the planner knows what to remove from the virtual pantry. Don't worry about telling the planner you ate foods that you never had in the pantry - it will ignore whatever it's not already keeping track of.