Backup your board & schematic first. Open the schematic but NOT the board. If the board is open, close it. Delete the jumper on the schematic. Now open the board & run an ERC. That 'shouldn't have fixed it.
In general with Eagle, once you have created a board from a schematic, never work on one without the other being open. If you open one, Eagle will ask if you want to open the other. Always say yes.
Eagle by default makes backups. Renaming earlier backup files to a valid (different) name may get you back to the point before you caused the problem.
Keith