Hi Emily,
Sorry, what a shame.
Just a thought.
Another member recently that Petco is having a dollar/gallon tank sale?

Consider completely rinsing the rest of the substrate out. Dry it really well ( no water should be there).
Then put another heavy coat of Aquarium sealant down - expanding over the the area/ overlapping what you have already done.

I don't know how big this tank is ? But for a dollar a gallon, you might spend more on aquarium sealant than on a new tank as you will be needing a lot to ensure it will not leak after all your hard work.

Lynn