Okay, I found out what happened with the $100. My account has not been cancelled.  It was below the required minimum balance when I deposited the $100, so cashing the $100 e-check would've brought it below the miminum again. They can't do that, so the check bounced. I have no idea why the credit card company said the check had been returned because the account was cancelled.

Still not happy, but I'm kind of relieved to know that my credit union hasn't cancelled my account (even though that's the least of my financial worries). I got my mom to write a $100 check for me and send it in to the CC company.