HP Pavilion PC turns on and off but won't connect to screen or mouse

[markas]

I have a HP Pavilion desktop pc and it switches on and off the little green light at back stays on i have sound of fan and prosseser fan i have checked all wires and reset it by holding power button for 10 seconds with cable out the computer definatly has power going in to everything

but the mouse should have a red light - but it doesnt and the screen doesnt come on now i know the screen works and the mouse does but for some reason it just won go together now i have had this computer for just over a year and it was working fine yesterday but today its like something just told it not to let me go on to anything

also the chips are in correctly and all wires are in correctly also
im out of ideas guys i have to ask for help please can someone help me

 

[Mark56]

First thing I would try is to unplug the keyboard and mouse, then switch on and see what happens, if no change reconnect them. If it boots up reconnect one at a time to find which one is the cause.

Go inside the box and remove all your RAM sticks and clean the contacts with a soft pencil eraser (up and down not from side to side) Then refit just one of the sticks and switch on. If no change swap the sticks and try again. If you only have one stick just clean it and fit it back into a different slot.

If still no change and you have a graphics card do the same with that.

Post back with the results.