One channel audio not working

Hello,

First let me give the details about my pc config.;

Processor : AMD Athlon 64 LE 1600
Motherboard : Gigabyte Technologies (M61PME-S2)
Chipset :NVIDIA MCP61
Memmory : Transcend DDR2 800Mhz
Graphics : NVIDIA GeForce 6150SE
Audio Driver: Realtech HD Audio

I have one 2.1 speaker system attached to my PC. It was working very fine before the problem starts. Now left channel of the audio is not working. My speaker system does not have any problem as i checked using my mobile phone 3.5 mm output; both the channels were working fine. What could be the problem? Please help me out. Waiting for your reply. Thanks in advance.