Suddenly thrown out of admin panel - Joomla! Forum - community, help and support

i have been thrown out of admin panel while working on own site. unable log in again. keep getting message "use valid username , password gain access administrator backend."

i have checked jm_users table in database , looks nothing has been changed there. have tried log on using other registered user names, doesn't work either... going on????

i'm running latest release of 2.5.

it has not been established far can see, doing before started error?
• is new installation?
  
• migration
• upgrading joomla 2.5.x 2.5.6 or updating extensions?
• adding extensions?
• uninstalling/deleting extensions?
• editing database?
• modifying acl permissions aspect of site or backend?
• changing template?
• editing core files?
• moving joomla_root/ directory , contents within host?
• changing host or moving local remote server?
• changing user authentication methods (modifying active plugin)
1. has host updated server?
2. please run forum post assistant provide information try , resolve issue.

also localhost or remote hosted site.