Disclaimer: I haven't had this happen at DLR, but I have had this happen at WDW. (I did have this happen with MaxPass in the old days, so I'm assuming it's the same.)
If you've scanned in, you have to wait in line until they decide to clear the line - which they won't do if they think the downtime will be brief. If and when they clear the line, you'll get an anytime LL from a CM - they'll scan your pass. (Depending on the ride, it would be usable on other rides as well, just not the ILL rides if it's a G+ LL.)
If you leave the line before that happens, you don't get anything. (The LL you had is already gone.)
Btw - If the ride goes down before the scan, you'll get an anytime LL automatically in the app.