You'll need a key, which happens to be in two parts. And finding both parts will take you quite a while, so try not to fret.
One half of the key is in Jessalyn's backpack, which is behind the double doors in the too room --
How do I open the double doors?
-- and the other half is in the crypt --
-- inside the snake coffin.
Once you have both halves, Nancy will put the key together. You can now use it to open the locked door --
-- or at least, to open the panel and reveal another puzzle. Blast! Read to the end of this hint for the answer to this puzzle.
This is the same type of puzzle as the one you solved in the fountain in the ruins, remember that one?
How do I solve the puzzle on the ruins fountain?
The only difference is this one doesn't have the numbers printed in. You can find the numbers for this puzzle in the crypt, in the bird coffin.
Other than that, the puzzle is the same. Oh, you had trouble with the first one too? Okay, here's the answer: