How to fix this ?

It's above ground?

When was the submersible pump put in?

Maybe it siphoned out?

screen-shot-2021-05-20-at-8-31-18-am-png.335129
Yes it is above ground. The pump was inside when I moved in, so I guess you're right
 
Look around for any punctures or rips.

If everything looks good, you can probably fill it up.

You might want to have a service check everything to make sure that it will fill ok with no problems.
 
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