Since you now have some healthy koi left, I suspect one of the koi viral diseases went through your pond.
Unfortunately, over the last ten years some viral infections have become common in koi. One called Spring Viraemia of carp (SVC) often flares up in the spring and the fall. It is the less common of the viral diseases, but has been found in the USA. Floating on the side is one symptom. There are others, but fish don't necessarily exhibit them. Sometimes the fish seem to die with no symptoms at all. There is no effective treatment.
Koi Herpes Virus (KHV) often produces lesions on the gills and skin, but there have been outbreaks where the fish suffered mostly internal damage and died before external signs developed. KHV is quite common in koi. It can remain dormant for a time, but becomes full-blown when certain temperature fluctuations develop. again, spring and fall are the times this may occur. This is another disease which can't be treated.
You are not the only koi keeper to suffer devastating losses like this. It is heartbreaking when it happens. I'm sure you are wondering what to do now. The KHV organism can only survive outside of a host for about 24 hours, so your pond is not contaminated if that was the problem. However, you have no guarantee that any new koi you purchase will be free of the virus. SVC survives indefinitely in water, so if that's what happened, any new koi could contract the disease. The tilapia should be immune.
Some of the places that sell more expensive koi put the fish through a special quarantine with temperature changes to induce the diseases if they are present. Infected fish are destroyed. If you want more koi, it would be best to buy from such a merchant. But even then, there is not a 100% guarantee.
As for the test kit, it would be good to get one regardless of what the fish dies from. Part of routine pond maintenance is testing the water once a week. Here is an online source:
If you need anything else. let em know. I wish you success with your pond from now on.