When you open the switch, the inductor starts charging the capacitor.
If H is large, and C is small, then voltage will rise very high. (This assumes there is no ohmic load involved.)
To calculate the first voltage swing, there are formulae, which involve joules. A formula for the inductor, and another for the capacitor.
Kinetic energy is converted to potential energy, then back again.
The equation has to do with:
(a) so many Amperes in so many Henries, equates to...
(b) so many Volts on so many Farads.