@Anonymous
Some Watch List issues can be solved by signing out and signing back in; if you have more than one account, make sure you are using the correct one.
Make sure you are using the general Watch List link rather than a subset of the watch list such as a particular category, or just the "ended" items of your list.
https://www.ebay.com/myb/WatchList
If the problem has to do with the Watch List page not loading properly, try holding down the shift key and refreshing the page; also try getting to your Watch List items using the Summary page or this "All Buying" link:
https://my.ebay.com/ws/eBayISAPI.dll?MyEbayBeta¤tpage=MyeBayNextAllBuying
If you need to reach your other custom lists, but are having problems with the Watch List page, try this "All Lists" link:
https://my.ebay.com/ws/eBayISAPI.dll?MyEbayBeta&SortOrder=ASCEND&GotoPage=1&MyeBayListIndex.Filter=All&View=MyeBayListIndex&tagId=-100&Column=TimeLeft&CurrentPage=MyeBayNextListIndex&ssPageName=STRK:ME:PAGE&StoreCatFilter=0&SubmitAction.ChangePage=x&FormatFilter=AllFormat
If none of that helps and all of your Watch List items are gone, see this thread for some ideas about recovering the results of recent searches and recently viewed items:
https://community.ebay.com/t5/Archive-My-Account-Q-A/My-ENTIRE-extensive-watchlist-disappeared/qaq-p/30724572
