I have only one domain on namecheap, but since a few month I cannot access it any more. There is a captcha service (hcaptcha) before it which totally locks me out.
You might be using a VPN that is blocked by Namecheap. Try turning off the VPN and then logging in. If this is inconvenient, you can transfer the domain to a registrar that does not block VPNs. As far as I am aware, Namecheap is the only one that does.