When you add a field to a form, a variable is automatically created to store the data that comes in. When you delete the field from the form, the variable still exists and the data will be retained. If you go to the form designer, in the pane with all the fields to choose from, there's a Variables tab at the top. If you click the Variables tab, you'll see a list of variables that are a part of the process. Any variable that is used on the current form will be grayed out, any variable not used on this form will be enabled. 

If you just deleted the field from the form, the variable should still exist in this variables tab and all the data will still be associated with it. You could even drag the variable on to the form again to bring it back and collect data again. 

If you don't see the variable here, the variable was permanently deleted and the data would be gone with it. This means that a process admin came to this Variables pane, opened up the settings gear, deleted the variable, and confirmed in the dialog that they wanted to deleted the variable completely.