A court that removes an executor must appoint someone else to take over the job. If the will names an alternate, generally the court would name that person to serve, unless there’s some legal reason the person can’t fill the post.
If the will doesn’t name an alternate executor, then the court will turn to state law, which will provide a priority list of those who are entitled to serve as executor. In most states, the surviving spouse is first on the list, followed by adult children and then more distant family members.