Hi,
Please check you config via CLI, and ensure following,
http://www.cisco.com/univercd/cc/td/doc/product/multisec/asa_sw/v_7_2/conf_gd/sysadmin/mgaccess.htm#wp1054101
"The security appliance allows Telnet connections to the security appliance for management purposes. You cannot use Telnet to the lowest security interface unless you use Telnet inside an IPSec tunnel.
The security appliance allows a maximum of 5 concurrent Telnet connections per context, if available, with a maximum of 100 connections divided between all contexts. "
Try to add ssh access, and see if you are able to ssh, when you get blank screen via telnet.
Regards,
Prem