my home pc will not power up?

when i switch on my pc at the mains and then press the power button it sometimes will not power up. A green light is displayed on the front to show power is on. I then have to remove the cover and unconnect and re-connect several wires and parts - then when i boot up again it works every time. The next day when i come to power up it will not boot and i get only green power light - so i have to repeat the removing case process all over again. Any ideas what is wrong ?