Sound does not work on your system. Prior to turning your system on it was working. What have you done to cause this and how can you correct it?
You checked to see if you disconnected any wiring and used a sound button to raise and lower sound on your monitor. You can view and go through some steps in a No sound in Windows Microsoft article. You can also perform a Windows system restore, if those steps don't resolve your issue.
Basically, you pick a restore point before your sound problem.
1. Log on to Windows as an Administrator.
2. Click Start, point to All Programs, point to Accessories, point to System Tools, and then click System Restore. System Restore starts.
3. On a Welcome to System Restore page, click Restore my computer to an earlier time (if it is not already selected), and then click Next.
4. On a Select a Restore Point page, click a most recent system restore point on this list, click a restore point list, and then click Next.
NOTE: A System Restore message may appear that lists configuration changes that System Restore will make. Click OK.
5. On a Confirm Restore Point Selection page, click Next. System Restore restores that previous Windows XP configuration, and then restarts your computer.
6. Log on to that computer as an Administrator. A System Restore Restoration Complete page is displayed.
7. Click OK.
Hopefully, this will fix your sound problem. These instructions were written for a computer running Windows XP.