LTSpice - voltage goes through the roof in LC circuit
To expand a bit on Tony's comment, LTspice has a default series resistance of 1m\$\Omega\$ for an inductor, so you have an initial current of 6,000A.
Aside from being pretty unrealistic unless the inductor and core were enormous, that means that there is 18,000 J of energy stored in the inductor, so one would expect a peak voltage across the capacitor of about 190,000 V from conservation of energy.