Not getting paid the amount promised in onboarding email

I have been working on the Diamond project since Sept. It has just come to my attention that I am not being paid per hour the same amount I was promised in my onboarding email. I have looked and can not find that email since it has been so long. How can I correct this? I have tried multiple ways (ZenDesk etc) already and always get the run-around to go to a different place. 

Thanks Krista