How long has it been since you worked? How long have you worked there?

Have you discussed this with the company?

This isn't really a legal problem. They can let you go for many reasons. It is actually to your benefit that they call it a resignation.

I find it odd that you received an email and have not been contacted by a manager by other means before this. You need to discuss it with them. I suggest actually going in and speaking with the boss. Don't play email tag.