@Emma_Lewis the first question, in fact the only important question, is this, ‘are your parents current VM broadband customers?’
Now if the answer is no, then there is a very good chance that the email address, mailbox and all the email within it has been, or is in the process of being deleted. There is a great deal of historical baggage, and quite a bit of misunderstanding attached to the old Virgin.net addresses, but they date back to the dial-up internet days. When VM decided to terminate this service along with the old ADSL internet connection, then users were given the option to take out a VM broadband subscription and keep the addresses otherwise they were given a one year (I think) grace period to make alternative arrangements after which the mailbox would be deleted.
Now this dates back to 2015/2016 and unfortunately VM didn’t always keep to their own timetable and deadlines and sometimes these mailboxes were left working in a sort of zombie state but are prone to being caught up in a periodic cleanup and deleted without further notice at any point. VM will ‘claim’, not without some justification, that users were informed back in the last decade and it isn’t their fault if people ignored it.
What I will do is escalate this to the forum team who will offer to get in touch and see what can be done. If the above scenario applies to your parents, then I am afraid that the outlook isn’t too good. Otherwise if they are VM broadband customers then it should be a case of attaching the email address to their broadband account.