Just wanted to relate this experience in case others find themselves in a similar situation (or have already gone through same).
I logged into my agent's portal (the old one) and all my HMRC services and client data had disappeared. Spoke to HMRC helpdesk and was directed to raise a support query using the "Get help with this page" at the bottom of the sign-in page. Fair dues to the HMRC online support team, I got a reply within 1 hr asking for some further details. When I then logged in to get these details, I found my services and client data there as before (BIG sigh of relief!).
Not sure whether it was my fault logging in somewhere I shouldn't have (unlikely as I access the portal daily) or just pure divine intervention, I just wanted to relate this for anyone who comes across a similar experience.