I lost my proof of purchase for my apple watch. i purchased a two year protection plan. Can i recover my proof of purchase?

if you can get the serial number for it Apple can email it to you.

How can you contact apple to give them the serial number?

Contact the place you purchased the device from. If an Apple Store, then contact that store.

I was able to see up until last week if I had Apple Care on my purchases not all disappeared! 😟 What the heck Apple?

Proof of purchase would be a receipt , if you bought it digitally then you will always have a copy of the recipe from the website you bought it from but otherwise its a bit tricky , as in you would have to talk to your bank find the transaction , (time and date) and from there use the transaction code (these are generated for every purchase you make) to prove you made a payment to the store you purchased the item from,

Thats all i can give , as i had a similar problem but online where the website never made a receipt but still charded me and thats what i had to do to prove i payed

Hope this helped