$\begin{aligned}
& \text { Velocity just before collision }=\sqrt{2 \mathrm{gh}} \
& \text { Velocity just after collision }=\sqrt{2 \mathrm{g}\left(\frac{\mathrm{h}}{2}\right)} \
& \therefore \Delta \mathrm{KE}=\frac{1}{2} \mathrm{m}(2 \mathrm{gh})-\frac{1}{2} \mathrm{mgh} \
& =\frac{1}{2} \mathrm{mgh} \
& \therefore % \text { loss in energy } \
& =\frac{\Delta \mathrm{KE}}{\mathrm{KE}_{\mathrm{i}}} \times 100=\frac{\frac{1}{2} \mathrm{mgh}}{\frac{1}{2} \mathrm{mg} 2 \mathrm{~h}} \times 100=50 %
\end{aligned}$