ProtonVPN got hit with an attack recently that wiped their database so there’s probably nothing there to authenticate to anymore.
Also a bunch of providers who claimed zero log policies got outed for… saving a ton of logs. So I would say take VPNs with a grain of salt. It’s honestly kind of a sleezy business. Check out wiregaurd and see if you can put together your own little proxy or something
Use the username and password in the OpenVPN / IKEv2 username tab of account section, which is available in https://account.protonvpn.com/login. this can be accessed through original credentials.