Haris Fitri, If the buttons have just started acting up since the drop, use the ifixit disassembly guide below, you can open up the device and see what is making the power/volume button not to work properly, could be dirty,dislodged from position/not seating properly or may need replaced from damage. Good luck. I hope this helped you out, if so let me know by pressing the helpful button.
[guide|23615]
https://flipsy.com/blog/14/05/How-To-Repair-A-Broken-HTC-One-Power-Button
https://www.ebay.ca/sch/i.html?_odkw=htc+one+m8+power+button&adpos=1o1&gclid=EAIaIQobChMIlK-PjcLP1AIVCAxpCh0gigOIEAMYASAAEgINCvD_BwE&ul_noapp=true&geo_id=34636&MT_ID=16&crlp=174863092766_1008&keyword=htc+one+m8+power+button&rlsatarget=kwd-83108755965&device=c&abcId=653836&crdt=0&treatment_id=7&clk_rvr_id=1238440723920&_osacat=0&_from=R40&_trksid=p2045573.m570.l1313.TR0.TRC0.H0.Xhtc+one+m8+power%2Fvolume+button.TRS0&_nkw=htc+one+m8+power%2Fvolume+button&_sacat=0