Validate NSX is showing a red dot in the HCX VAMI, and it is failing to login to the NSX-T UI with the admin credentials. If both of these are validated, then proceed with resetting the expired admin password using the following document -
https://docs.vmware.com/en/VMware-NSX-T-Data-Center/3.2/administration/GUID-8816B842-2EC4-40A8-A618-F68DB29FABD2.html.
Note: If root is also expired, the process under "Procedure" can be followed in that same document above.
Once the admin password has been reset, login to the HCX VAMI (on port 9443) and edit the current NSX credentials and add the new admin password.
If it fails to add the password, please delete the current NSX registration, reboot the HCX Manager, and re-register NSX in HCX using the same NSX URL, admin and the new admin password.