Is the Diels Alder reaction reversible?
Consider the mechanism shown here. You'll see this:
The regular Diels-Alder reaction, despite usually being heated, occurs at a lower temperature than when the reverse direction is favored, and in these reaction conditions, the equilibrium favors the product.
Now try reversing that mechanism.
All you have to do is heat the product even more. Then the resultant retro-Diels Alder reaction favors the diene and dienophile (the reactants).
Conclusion: The reverse Diels-Alder is favored at higher temperatures, and the forward Diels-Alder is favored at lower temperatures.
CHALLENGE: Can you draw the reverse mechanism? Hint: It's a concerted "cyclic" flow of arrows.