Disabling some of the subdomain

I have a WordPress Multisite. I created some subdomains that worked correctly.
for example:
(a.example.com , b.example.com , c.example.com)

But suddenly two of the subdomains disabled and I received a message “server not found” for these subdomains (a.example.com , b.example.com), But other subdomain (c.example.com) works properly.

To be able to have access again to the subdomains (a , b), I had to change my name subdomains to (a2.example.com , b2.example.com).

Please tell me how can i get back to the primary name of subdomain with the name of (a , b).

Thanks in advance