You mean it is physically lost, your it is just not detected?
Go to Start -> Control Panel -> System -> Hardware -> Device Manager, then see if the sound or audio list has yellow or red flags.
If it has flags, you need to go to the website of the computer manufacturer or the sound card manufacturer to download and install new sound drivers.
Otherwise, take the computer back to the people who did the reformat and ask them to re-do it correctly... It should take them no more than a half hour.