Does resonance increase energy?

Hint: Chemical compounds exist in resonating structures. These structures are known as canonical forms, and they do not have any real existence.

Complete answer:
Resonance form or structure of any molecule is known to be used when a single Lewis structure cannot describe the molecule accurately. The resonance structures have the same energy and, same position of nuclei. The resonating structures are of no real existence, as any molecule cannot exist for more than a fraction of time.
Resonance can be explained by the fact that delocalization of electrons in a compound, results in various canonical forms of a molecule. The delocalization is shown for a carbon dioxide $C{{O}_{2}}$ molecule as,
\[\left[ :\overset{\oplus }{\mathop{O}}\,\equiv C-\underset{\centerdot \centerdot }{\overset{\overset{-}{\mathop{\centerdot \centerdot }}\,}{\mathop{O}}}\,:\,\leftrightarrow \,:\ddot{O}=\ddot{O}:\,\leftrightarrow :\underset{\centerdot \centerdot }{\overset{\overset{-}{\mathop{\centerdot \centerdot }}\,}{\mathop{O}}}\,-C\equiv \overset{\oplus }{\mathop{O}}\,: \right]\]
The 3 resonating structures of $C{{O}_{2}}$, shows the delocalization of electrons. Through this delocalization the molecule gains stability. The molecule gains stability because the energy of the resonance hybrid structure is less than the energy of any of the canonical forms.
The delocalization of electrons is accompanied by the reduction in energy, more the delocalization of electrons, more, the reduction in the energy is observed.
Hence, resonance does not increase energy; it increases the stability of a molecule and decreases the energy.

Note:
Resonance structures contain the same number of bonding, and non bonding electrons as in the Lewis structure. The resonance structures show the shifting of electrons, and can be seen in compounds containing double bonds. More the number of resonating structures a molecule has, the more stable the molecule is considered.