Maybe it got caught on something in the tank.
After the tank lights go out, shine a torch on the fish and see if it has a yellow or gold sheen, that would be velvet.
In the mean time, do a 75% water change and gravel clean the substrate every day for a week. This will dilute any disease organisms in the water and provide a cleaner tank for the fish to heal.
Make sure any new water is free of chlorine/ chloramine before it's added to the tank.
Clean the filter if it hasn't been done in the last 2 weeks. However, if the filter is less than 6 weeks old, do not clean it. Wash the filter materials/ media in a bucket of tank water and re-use them. Tip the bucket of dirty water on the garden/ lawn. Cleaning the filter means less gunk and cleaner water with fewer pathogens.