@angus1x
I have heard of one case of a user who said their watch list "disappeared" and came back -- I do not know what exactly happened there. Perhaps the user was signed out of their account or an eBay glitch prevented the page from loading. Occasionally when I load my watch list page it fails to load completely or comes up blank; in that case refreshing the page is usually enough to restore it.
I do not think it is possible to delete your entire Watch list the way it is possible to delete other custom lists. But it is very possible to delete all the items on a list page by checking the "select all" box at the top of the page and then hitting delete. If you have inadvertently deleted all of your items that way, I do not know of any way to restore them.
If you are faced with that latter situation, your best bet may be to check your recently viewed items and see if any of them are from your Watch list:
https://www.ebay.com/myb/container?key=recentlyviewed
You may be able to recreate some of your most recent searches, and find some of your watched items that way. See here for some techniques to try to recreate your searches:
https://community.ebay.com/t5/Member-To-Member-Support/search-history/qaq-p/30192652
If by chance you had an open tab with your watch list previously loaded on another computer which has not been refreshed since you lost your watch list, you might be able to use that to open individual items in other tabs and add them back to your watch list, but you will not be able to change the page number and care must be taken not to refresh the list page lest it reload the empty list.
