when i tried to reply i found out that is locked by admin.
Threads can be locked by the creator or by admins. If the thread was locked by an admin, you were sent an e-mail or PM explaining why.
And how can it be unlocked?
If you accidentally locked it, you should be able to unlock it. If an admin locked it, and you have resolved the reason for the thread being locked, the admin who locked it can unlock it.
Click the "Report to moderator" link, if present to ask that it be unlocked.