I need one more reference letter but

So I was asked to provide 3 reference letters by tomorrow. I have 2 out of 3. I only texted my old internship supervisor since honestly I wasn’t sure if I should call, text or email her for the favor. She never got back to me and Idk who else I should ask. The employer asked for 3 reference letters from my previous employers. Idk who else to ask since IDK my previous supervisor doesn’t work for the company anymore. I am freaking out. This is a real serious job. I just finished college and don’t want to mess up on getting this job. Idk if I should just explain the situation to the hiring manager or what?! I currently work at a call center and my work trainer and I had close contact but she wasn’t a supervisor but that’s like my last resort.

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