[ubuntu] Lock screen -- screen goes blank

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http://ubuntuforums.org – I have a HP Laptop loaded with 9.10 64-bit. If i do "Lock Screen" , after some time the screen goes completely blank and it won't show the login screen. I tried by pressing multiple keys, but i am not getting the login screen. Hard rebooting is the only for me to get back into my work. Please anyone tell me how can i solve this issue ? 7.04 ,7.10, 8.04, 8.10 ,and 9.04 were perfectly working fine. I am facing this issue only with 9.10. (Hardware)