You've probably snapped a spring. No special tools needed. Take cassette off, pull off little metal axle end (drive side) with mole grips/whatever. Pull lightly on freehub bodyand twist anti-clockwise slightly. The freehub body will come off in your hand, hopefully with pawls/springs intact. Clean it all out, check springs and pawls. If you ain't snapped a spring you've probably done the freehub shell bearings in. Check them out and replace if neccesary. Once its all cleaned out lube with light oil, push freehub back onto axle. Use screwdriver to ease pawls into place. You'll need to seat the green seal so use a screwdriver to gently press it back in place. Put axle end back on, then cassette and lockring. And there we are.