Ubuntu getting frozen with '{ DRDY ERR }' error

Hi,

I am using Ubuntu LTS 12.04.

For last few weeks it is getting frozen on INIT 5 and and I can see follwoing logs in single user mode.

Have any body any clue for the following:

Feb  3 09:14:07 CB11WS1 kernel: [  493.735118] ata1.00: status: { DRDY ERR }
Feb  3 09:14:07 CB11WS1 kernel: [  493.735121] ata1.00: error: { UNC }
Feb  3 09:14:07 CB11WS1 kernel: [  493.772387] ata1.00: configured for UDMA/133
Feb  3 09:14:07 CB11WS1 kernel: [  493.772411] sd 0:0:0:0: [sda] Unhandled sense code
Feb  3 09:14:07 CB11WS1 kernel: [  493.772414] sd 0:0:0:0: [sda]  Result: hostbyte=DID_OK driverbyte=DRIVER_SENSE
Feb  3 09:14:07 CB11WS1 kernel: [  493.772420] sd 0:0:0:0: [sda]  Sense Key : Medium Error [current] [descriptor]
Feb  3 09:14:07 CB11WS1 kernel: [  493.772426] Descriptor sense data with sense descriptors (in hex):
Feb  3 09:14:07 CB11WS1 kernel: [  493.772430]         72 03 11 04 00 00 00 0c 00 0a 80 00 00 00 00 00 
Feb  3 09:14:07 CB11WS1 kernel: [  493.772443]         2f 92 7e d8 
Feb  3 09:14:07 CB11WS1 kernel: [  493.772449] sd 0:0:0:0: [sda]  Add. Sense: Unrecovered read error - auto reallocate failed
Feb  3 09:14:07 CB11WS1 kernel: [  493.772456] sd 0:0:0:0: [sda] CDB: Read(10): 28 00 2f 92 7e d8 00 00 08 00
Feb  3 09:14:07 CB11WS1 kernel: [  493.772469] end_request: I/O error, dev sda, sector 798129880