Resonance structure of cyclobutadiene?
Antiaromaticity is a concept describing an explicitly fictional situation, no molecule is antiaromatic, because all molecules will react to avoid the situation.

So the answer to the implied question is that cyclobutadiene has very little resonance because the molecule is prevented from being antiaromatic (by the nature of the universe and its physics).

Elaboration: 
To avoid this situation, butadiene is very reactive; it will undergo an intermolecular Diels-Alder reaction avoid this antiaromatic state.  When reaction is prevented something much more interesting happens.  When frozen in an argon matrix, x-ray crystallography of cyclobutadiene indicates the molecule is highly rectangular, not square.  For dienyl resonance, there is some changing of bond lengths.  In butadiene, the 2,3 bond shortens.  If this were to occur in cyclobutadiene, then there would be degeneracy in the orbitals leading to a Frost diagram that would describe an antiaromatic state.  So if reaction is prevented, the molecule distorts its own geometry. 

The fictional resonance structures you would draw are double bonds on the top and bottom, and double bonds on the sides.  However, generally, one might assume there is no resonance for this molecule.