Sharepoint List reset after 20000 Items

Hi,

I have a Sharepoint list that used to contain about 20000 items but after I have came back after Christmas the list seems to have reset and now I am back on 400 items. The list gets populated by a PowerApp and it will have been updated over Christmas. I am wondering where the other items have disappeared to. Have they been automatically deleted and are they recoverable. I do have versioning switched on but cannot find anything about restoring the whole list to a previous version just specific items. 

Thank you in advance for your help. 

Edit - Just to add I had a flow to calculate the total number of items it seems like it 12987 items and then reset back to 0 items so it has completely cleared the list. I don't get why it would delete the contents of the list as nothing in the PowerApp or Flow would tell it to and I have seen a lot of users working with more items then me. Will I still be able to get an export of the list before it got cleared.