Please help us to help you by reading and following the posting guidelines located in this thread: Please read before requesting assistance. The more information you give us the better we will be able to assist you. Thank you.
Please post a screen shot of your Environment details (see Admin panel -> Dashboard -> Information)
Please consult your [unknown webserver], PHP, MySQL error logs to see if anything is being logged and post those errors here.
Is the ticket in a department that your Agent account has access to? [note: tickets with referral access do not let you close them with out access to the department that the ticket is in.]