You'll need to go into the list setting and remove the change.
click the cog (flower looking thing) at top right and find "List settings" menu item. It will bring up a list of column fields, go into the field and edit it so as to remove what you changed.
Then you need to figure out what the issue was. What type of Column did you add? or what was the change? Changing a columns type can cause all sorts of issue. Perhaps create new column of the new type and see if that works.
