First go to the three dot menu next to the assignment title in grades, and select "hide grades" to limit student confusion. If you mean that you accidentally set the default grade, you can manually delete it. You could also repeat the steps to set the default grade to zero (if that isn't what you set it to already), and then immediately open the speedgrader and apply a real grade to all actual submissions. Replacing the grade you have already entered will require checking the box for overwriting existing grades.
If what you did was accidentally overwrite your existing grades, you can find the history of what they changed from in the gradebook history. On the grades tab, the gradebook history is one of the options in the drop down menu title gradebook.
Once you have the grades restored to the way you want them, you can go back to where you hid the grades and this time select post grades to reveal them to students
