@ probus
I'd like to think we could get all our Likes back from 4th April when it went down, but if they weren't registering on a database, then they're lost. My Likes are back, but apparently the problem is not yet fixed for you.
What the Notification is telling you (in reality) is that you've received no Likes since 24th April when the system started working again. No one received Likes between 4th and 24th. Have you checked your posts of the last few days to see if you've received any likes. The other thing you can do is to Like someone's post, and that should (unfortunately, I think; and unless it has been changed) show in Notifications.