C720 turns on, but is really dark and brightness won't change.

I know this is a really old thread, but it turns out this is really easy to fix on both the C720 and C740.  I've fixed probably a dozen of these over the last couple of years at our school.  

The problem is that the connector between the motherboard and display panel comes loose.  You have to carefully remove the display bezel (see youtube videos such as this one: https://www.youtube.com/watch?v=yOjkMZR47AY) and then remove the 4 screws holding the display panel in.  Carefully flip the display down and re-seat the cable on the bottom of the back of the display panel.  I've found that it helps to remove the adhesive tape and re-secure it more tightly (creating tension that tends to pull the connector together, rather than pulling it apart).  Reassemble & resume use.