Visit the store that sold it to you and try to work out a return with them, after all, they sold it. A phone call is just not as effective as a personal appearance (with the monitor). Bring everything with you including the original box and all manuals, software... everything or they'll deny the return based on missing items.
You may need to get a RMA from LG and send it to them. Make the store assist you with that if they don't take it back themselves.