Macbook Pro will no longer detect external display

I have had a similar problem (albeit with a Powerbook) and I had to change my DVI-VGA (back then we didn't have Mini-DP). It was working until one day it simply stopped working. Since I had a lot of displays at home, I tried -like you- different cables and displays to no avail until I found another adapter and boom, it worked.

Another way to test this is go to an App Store or similar with your machine and your cable. Try to make it work there. If it doesn't, they will have extra adapters to rule out that possibility. If a new cable, display, adapter also fails, then your Mini-DP port could be failing and that requires an App Store anyway…

Try resetting the parameter RAM using these instructions

You can also try resetting the SMC using these instructions