I bought Warhammer 40,000 Rogue Trader during the winter sale but for some reason the game's store page doesn't show I've purchased it and its not appearing in my list of orders. Obviously I don't want to have to buy t again someday f something happens to my computer so this has me very concerned.

I know this is almost certainly not the issue, but just to rule it out, you did check both versions right? There is a "Standard Edition" and a "Deluxe Edition" and if you own one but looked on the store page of the other, it won't show as owned. You mentioned it's not showing up in your orders, but is it still showing in your game library?

Winter Sale was a long time ago.
You only notice now?

Normally, I would say: if it doesn't show up in your orders, it has not been bought.
What about your email receipt?
Bank statement, resp. PayPal email?
Those are also missing? What has that to do with anything?

The game shows up in your public profile with nearly 85 hours of playtime and several achievements.
Just check your game library and order history again. Maybe a typo prevented you from finding it.
Also, when you buy something it's yours "forever". No need to rebuy anything ever unless you want to gift more copies to your friends.

Actually that was it I was looking at the standard edition because I had forgotten that I bought the deluxe edition because I very rarely buy deluxe editions. Usually I just buy the standard, try the game, and then decided if I want the deluxe upgrade. Thanks

My order history is still weird though. If I type something into the search orders area I get a no results message but I can find things if I know the correct page number or just scroll through my order history until I find it.

The order history search is currently buggy. GOG knows about and is working on it.

The important thing though is that you found your game. :-)