Now, it is can be bit  difficult to get your hall ticket number, but you can try by checking your E-mail there will be some mails from the exam conducting authority  and some information so you can get this or you can visit the official website. You can may be look for a forgot hall ticket option if there is or something like that  option or you can directly contact the exam authority .Most importantly don't lose your important docs because without them you won't be able to claim any hall ticket. You can also try by contacting your institute through which you gave exam. Hope it helps..