Why no sound?

Clarifications

Open the Control Panel then Sound and Audio Devices and then when the next window opens click on the Audio Tab and make sure that the correct Sound device is listed. If it's not click on the drop down arrow on the right hand side and then select the Realtech listing.

See if that restores the sound. If it doesn't post back with your OS type and so on and someone here will attempt to help.

Col

Probably some malware and/or virus. If system restore is enabled, restore to a prior point in time and that should put things right. Otherwise, a package like Malware Bytes should be able to spot the bug.

Delete the sound drivers then reboot your computer.
When up and running reinstall your sound drivers or update them if you have an add on card from another maker other than your motherboard.

Sounds like it could be a rogue application. There are tons of them, pretending to be legitimate apps and begging for your credit card data. If you know the name of the application (from the dialog it showed you, or if it's in your SysTray beside the clock), an Internet search should reveal instructions on how to remove it. Otherwise, I'd search for it using descriptive terms, like the text of the warning message it gave you. If you don't remember that either, try using more general terms to find it being discussed in a forum.