Help! Where did my money go?

My son sent me $750 in my Apple Cash. I transferred $700 into my regular bank account. No extra charges occurred, because I did not use instant transfer. After the transaction has gone through, the balance on my Apple Cash was clearly showing $50. After a week, my Apple Cash is now showing no balance. In fact it’s blank, asking me to set up my account. And every time I try to do that, it takes me to a dead end. I have no subscriptions through Apple. How can I get my $50 back on my Apple Cash? Help, please!