If your employment is terminated, your L1 Visa is automatically void. Your stay will be illegal. You have to 1) keep you job 2) your company sponsor you to apply for EB1 immigrant visa, 3) apply for adjustment of status.
If your employment is terminated, you have no base for step 2). Depending on your situations, there may be some other options for you to continue your stay.
Our firm specializes in Employment-based immigration. You are welcome to consult us.