I changed the housing on my D500 and the keypad circuit board as the menu key was sticking and it was a very tatty phone..the phone was full working, back light and all before i did this...i falowed a video on how to take the D500 apart which i falowed to the letter.. put it all back together no prob, now all buttons work BUT i have no back light at all...the key pad lights up but NOT the screen any ideas what i have done please..thanks

tryed to sort this now phone will not switch on at all, but it charges only HELLLLPPPP

Well the first obvious problem is that when you have opened up the phone to change the Circuit board for the keypad, that way you may have damaged one of the connection ports to the screen. The phone is not designed to be opened up and to be self repaired. First thing you should always do if your phone has any type of problem that is a risk to fix yourself is to return to the place you first purchased from or to a phone repair shop and if you pay a little bit more they can fix it for you with no problems as they have special equiptment to do this with.
Now that you have opened it by yourself to try to fix the problem i dont know what can be done.