What's gonna happen - 2

When an uncharged capacitor of same capacitance is connected in parallel , the capacitance is doubled

$Cp = C + C = 2C$

But the charge is equally distributed between the two. Hence charge on each capacitor is half its original value.

$Capacitor~Energy$ $=$ $U$ $=$ $\frac{1}{2}$ $\frac{Q^{2}}{C}$

Substituting $Q = \frac{1}{2}$ we get the answer as $\frac{1}{4} U$ .

Hence the value of $Capacitance~Energy$ becomes $one-fourth$ its original value.