Heat bed stop working

Hi,

I have been using Robo3d for sometimes and it seem that the heatbed often cuts off when its started printing. Funny enough, it can stay at 85c as long as it wants before it actually started printing.

When printing, the monitor in Repetier for the bed will be at 0.0c and it will stay there until the print is finish. I would have to hit the home button for the print after the job to properly start the print again and the graph would then jump back to around 30c.

Last night, I was printing for 6 hours and everything seem alright, the bed cut off as usual and it stays at 0.0. However, I couldn't re-activate the bed by hitting to home button anymore and the temp now stays at 0.0 with an occasional spike from times to times

here is the image of it. Help me!

 

ok, I found the reason. Loose wire.