Calculating inductor voltage in a second order linear circuit
Energy storage in an inductor is in magnetic flux, which is related to the current through the inductor. 

Therefore, the initial conditions for the circuit are given by the capacitor voltage and the inductor current. If you're only told "the switch has been closed for a long time", then you must determine the steady state of the part of the circuit with the switch closed. And realize that neither of the initial condition variables (\$I_L\$ and \$V_C\$) can change instantaneously --- their graphs will be continuous in time.

Once you know the two initial conditions at t=0, then you can use Kirchoff's laws to find the current through R1 and R2 after the switch opens, and these will give you the initial inductor voltage.