It sounds like someone didn't get the message, which is unfortunate. Someone's mistake or lack of follow-through cost you your job.

Sadly, in a situation like this, the burden of proof is on you. In the future, should you need to be out, follow up a call with an email. At least then you have documented proof that you reached out and notified somebody.

Hopefully you were/are able to find work with a different company.

https://www.thebalance.com/what-does-employment-at-will-mean-2060493