New online account means lost access to old info

When my husband tried to access his old online account he said he got a message that it had been deleted (I think he means deactivated) So in his frustration re-registered for a new account which he has access to - but how does this affect it if he needs to do a new tax return e.g. would he have to re-register for Self-Assessment if one was required or should it still be able to find a link with his last completed tax return if given the UTR number? I still get a self-assessment tile on my online account although I haven't needed to do a return since 2016

Hi,
If he needs to complete a Self Assessment then he can register here:
Check how to register for Self Assessment
This will re-open the old account to submit the tax return. 
Thank you.