This means the mIRC Lock options are set:
ALT+O > Other > LockClick the 'Unlock' button, type in your password and the screen will be activated. Uncheck the boxes 'Send' and 'Get' and for DCC Chat, 'Private Chats'. You may then want to relock mIRC depending on what your options are.
If you have forgotten the password, then see
this page for some help. Do be careful, and heed the warnings. If you did not set the lock and someone in authority (say, parents, teachers etc. did it) then there is little point trying to evade it as it can just be reset and could lead to your total banishment of using mIRC, depending on your situation - and I'm sure you don't want that!
Hope this helps

Regards,