No volume control means sound card/drivers are disabled/not installed/not working (Yes, very broadband!)
Has anything else changed, such as the date or time? I noted that you say the machine is 4/5 years old. This could mean that the battery CMOS battery is weak. This can show up as corrupt/default BIOS settings or the machine just playing dead.
From what you say, the sound module is integral to the motherboard, so the board number would be handy.
Re-start the machine and watch carefully for a message at the bottom of the screen that says something like 'Press <DEL> to enter setup'
Press the delete key when you see this message and wait for the menu.
Scan through the menus for a setting that says 'Onboard Audio' or similar. It might be under the advanced chipset menu or Peripherals and Feature setup. This should be set to enabled. If not, use the relevant keys listed to set it to enabled. Press escape to return to the top menu. If you have changed the setting, press F10 to save and exit, otherwise press escape to exit and not save.
Next, Restart machine to Windows. Right click on My Computer and click on properties. Click the device manager tab and expand the sound, video and game controllers folder. Anything look strange, like a question mark or cross against any of the listed values. Click on the properties for each to see if there any conflicts.
Finally, have you got a disk containing driver?
It's a bit long winded, but without actually seeing the machine...