I have attempted to sign into Splunk with both my Trail login credentials (admin as well as user) in addition to trying to sign in with the Default free creds (admin / changeme) and am unable to sign in.
I'm not really sure where to check next to resolve this, other documentation indicated that I should have been prompted to upgrade/convert my license on expiration of the trial...
If you move the password config file $SPLUNKHOME/etc/passwd file to $SPLUNKHOME/etc/passwd.bak. If you perform a restart of splunk you should then be able to access splunk with the default admin/changeme username and password.
Let me know if this does not work...maybe there is a new change to splunk that stops access as well?