Yesterday my blueyonder account spontaneously moved about 2000+ emails from my inbox to my spam/junk folder. When I opened my email on my phone, they were just moved. When I opened my email on my laptop, I could actually see the number on my inbox dropping by the second as they moved themselves. This effectively moved 4 years of emails!
I assume that your phone and the VM webmail system are connected via IMAP. Your experience is invariably the result of someone hitting the delete button on the phone’s email app (possibly accidentally), and misunderstanding that any delete action on the phone is replicated on the VM website. The suggestion is that you have selected ALL before the delete.
You should login to webmail and move the emails you want to keep back into the Inbox asap.