The particle is moving in a horizontal circle, so it is accelerated towards the centre with magnitude rv2. The horizontal force on the particle is due to the spring and is given by kx, where x is the elongation and k is the spring constant.
kx=rmv2=mω2r=mω2(l0+x)
⇒(k−mω2)x=mω2l0
⇒x=k−mω2mω2l0