Pool won't empty even when the sump is fully open

[Taylercorkett94]

This is going to sound stupid but when I was vacuuming my pool I opened the sump 1/3 of the way, when I was finished I forgot to close the sump and woke up to find my pool half empty. Now I've realised that s few pool lights need replacing and it could do with a clean soo I decided to empty it but it won't for some reason even with the sump fully open the multi-port on waste and the backwash valve open anyone know a reason why, thanks

[cpo2go]

It's hard to say, for sure, without being on-site but, it sounds like it is one of two things.
Try plugging your skimmer(s). This will divert all of the suction to your main drain at the bottom of the pool. You may be sucking too much air through your skimmers and causing your pump to cavitate. Or...
It could be that your main drain is capped or in any other way not connected. Attach a vacuum hose to your skimmer and sink it to the deepest part of the pool and turn your pump on. Make sure to plug any vacuum port besides the one you're using. A tennis ball makes a great plug for skimmers.