The default behavior of vCenter server is to expire your password after 90 days. You can configure this duration, turn it off, have it notify you that your password is about to expire, or provide alternate means of authentication.
If you don’t make some sort of change however, you may find after your server has been running for months that following a reboot you suddenly can’t login. (I also hope your password is not still ‘vmware’, someone will guess that).
Get Back In
The first thing you need is to regain access without re-installing and setting it up again. There is a VMware article that addresses this. Just look at the second part about resetting the password, the section at the top is not necessary since you can change it from the admin web interface.
Also, if you already have a LiveCD with the filesystem mounted you should be able to just remove the ‘x’ it prepends in the shadow password file for the user and be able to login again.
Change the password settings
After rebooting you can connect to the web admin interface (https://<yourmachine>:5480)
Login with your now working L/P and proceed straight to the Admin tab.
The easiest change you could make is to just turn off the expiration. Really strong passwords that are kept secure do not need to be rotated. If that’s your policy however, you should set it to remind you when it’s close to expiration by sending an email. You will need to setup email delivery before that will work, however.
The mail settings are in the vSphere web client (https://<yourmachine>:9443/) or the standalone client app under Home > Administration > vCenter Server Settings > Mail
Once you have inserted the mail server information you can add an email on the server appliance screen you already visited and you should receive the notifications.