I was offered a job , signed the offer letter, worked two days and now they are saying they can not proceed.

Nevada is considered right to work state: That means your employer can terminate you for any reasons, no reasons or good reason, and the same you can do. Unfortunately, your rights are not protected unless you were discriminated because of race, gender, religion, national origin and few others. You just signed the offer, and it was not a bilateral contract between you and the employer.