  We use high-resolution images of the sun obtained by the SDO/AIA instrument
to study the evolution of the coronal loops in a flaring solar active region.
During 15 February 2011 a X-2.2 class flare occurred in NOAA 11158, a
$\beta\gamma\delta$ sunspot complex. We identify three distinct phases of the
coronal loop dynamics during this event: (i) {\it Slow rise phase}: slow rising
motion of the loop-tops prior to the flare in response to slow rise of the
underlying flux rope, (ii) {\it Collapse phase}: sudden contraction of the
loop-tops with lower loops collapsing earlier than the higher loops, and (iii)
{\it Oscillation phase}: the loops exhibit global kink oscillations after the
collapse phase at different periods, with period decreasing with decreasing
height of the loops. The period of these loop oscillations is used to estimate
the field strength in the coronal loops of different loop lengths in this
active region. Further, we also use SDO/HMI observations to study the
photospheric changes close to the polarity inversion line (PIL). The
longitudinal magnetograms show step-wise permanent decrease in the magnetic
flux after the flare over a coherent patch along the PIL. Further, we examine
the HMI Stokes I,Q,U,V profiles over this patch and find that the Stokes-V
signal systematically decreases while the Stokes-Q and U signal increases after
the flare. These observations suggest that close to the PIL the field
configuration became more horizontal after the flare. We also use HMI vector
magnetic field observations to quantify the changes in the field inclination
angle and found an inward collapse of the field lines towards the polarity
inversion line (PIL) by $\sim$ 10$^\circ$. These observations are consistent
with the "coronal implosion" scenario and its predictions about flare related
photospheric field changes.
