Hi!
D:lines are a sort of ban to reject connections from a specific origin.
Maybe EFnet received some attacks from people using the same internet provider as yours and the IRC oper/admin banned them using a too general/large IP range or hostmask.
Anyway I suggest you to visit the
EFnet web page or
the EFnet forum and try to contact an EFnet IRC admin.
D:lines are usually purged every 24h on big networks.
Moreover they only applies to a local server, so try to connect to another EFnet server:
list of linked servers (BTW: it's not a mIRC-related problem)
Good luck