OK. here is all actions I have done. see if it can brings any ideas.
btw: I am not rash at all. just want to figure out this type of issues.
Because I am pretty sure it do happen a lot.
I try to make domain works without www this morning.
I go to General > wordpress site url (I change the wp url to the domain without www). suddenly, all the links stop working which includes admin login page.
Then, I go to mysql database, change the url back to www. meanwhile add htaccess make all the urls point to www.
Everything was working!! I can logon to the admin page. and I did one plugin update. (I am pretty sure admin page works after I update)
I didn't clear my cache since last year. so I decide to clear the cache.
After clear the cache. admin page won't left me in. I think that's all.
Thanks in advances.....