Nothing will happen, you're fine. If your UTXOs were already showing up in Postmix but were still unconfirmed, that means their current Whirlpool round was over and they didn't start the next one yet, so you didn't even disrupt a round by going offline.

When you connect Sparrow to your server again, you should see your UTXOs confirmed an ready to get into a new coinjoin round if you so desire.