Does the receiver make sound when you use a different source than the TV such as the built in FM radio?
Do the TV speakers make sound if you turn them back on?
Did you check the settings in the TV audio menu to make sure they haven't reset themselves?
Did you try unplugging the receiver from AC for a few minutes and plugging it back in?
If the receiver works with other sources then check the TV audio menu. Maybe it needs to be reset to output audio again.
If it doesn't then the receiver may have been damaged by the lightning. Some insurance will cover this kind of damaged.