Capitan upgrade has broken my secondary monitor display

My Mac Mini used to work fine with my 2 monitors (extended desktop style), but now the upgrade to El Capitan has caused the 2nd monitor to no longer be recognized by the OS. I'm thinking that probably the new OS doesn't have the correct drivers to handle external monitors. Does anyone know how I could update the system to work properly?

Here are some details:

Mac mini:

Model Identifier: Macmini7,1

Processor Name: Intel Core i5

Processor Speed: 2.6 GHz

Number of Processors: 1

Total Number of Cores: 2

L2 Cache (per Core): 256 KB

L3 Cache: 3 MB

Two Monitor details:

Brand: AOC

model: 2252W:

Resolution: 1920 x 1080 @ 60 Hz

Pixel Depth: 32-Bit Color (ARGB8888)

Display Serial Number: EUPE81A006479

Main Display: Yes

Mirror: Off

Online: Yes

Rotation: Supported

Connection Type: Analog VGA or Analog Over DVI-I

Adapter Firmware Version: 1.21

By the way, when I click on the "Displays" preference panel and press the Option button, it shows the "Detect Displays" button, but when it click the button the screen flashes for a moment and then nothing changes. It still does not recognize that a secondary monitor is connected and ready for use.

Any suggestions or information would be greatly appreciated!

Thanks!

Model Name: Mac mini

Memory: 8 GB