Old Chats disappearing from Whatsapp
My guess is, that you archived your chats. To find your archived chats, scroll down to the bottom of main WhatsApp page and you'll see the archive tab. Open it and your chats should all be there. Long press on any of them and move them back.

You can also try searching for a word that you might have written to a contact. For example, if you search for the word "thanks" in WhatsApps main search bar, that should bring up any chat that the word was written in.