This is a jukugo word that uses the on'yomi readings of the kanji. You didn't learn this reading for 厳 yet, so here's a mnemonic to help you:
To help you understand the solemnity of a moment of silence, usually a gong (ごん) will play a few times. As you listen to the echo and silence that comes after it. The gravity of what you're being silent for really sinks in.
The ceremony was held solemnly.