When the capacitor is connected to a d.c. source and then disconnected, it gets charged and then it starts discharging through the inductor. An induced emf is produced in the circuit which opposes the growth of current in $L$. When the capacitor is fully discharged, the electric energy stored in the capacitor (i.e. $1 / 2 q^{2 / C}$ ) gets converted fully into magnetic field energy $\left(1 / 2 L I^2\right)$. As soon as the discharge of the capacitor is complete, current stops and the magnetic flux linked with $L$ starts collapsing. Therefore, an induced emf is again developed which starts recharging the capacitor in the opposite direction. When the recharging is completed, all magnetic energy stored in $L$ appears as the electric energy between the plates of the capacitor. The entire process keeps on repeating and the energy taken from the cell keeps on oscillating between $C$ and $L$. Hence the current in the circuit starts oscillating as soon as the d.c. source charging the capacitor is disconnected.