When you came to the US, they gave you a H1 visa. Whne you entered the US, you got your I-94 with an expire data of your H1.
However, your employer should know when your H1 expires as they should have a copy of your I-94 from you. Did they talked with for the expiration? Did they renew your H1 visa with USCIS? If your H1 has been rewed, you should have a new I-94. The employer may just give you a copy of the new I-94.