my microphone isnt working ihave dell optiplex 7070

What kind of mic? 

Your system requires the Dell Realtek sound driver and the Waves Maxxaudio app for proper device detection 

DownloDd and reinstall both

https://www.dell.com/support/home/en-us/product-support/product/optiplex-7070-desktop/drivers

7070 models are desktop units.

Newer Dell models have TRRS mic/headset input.  There is no built in mic.

https://www.amazon.com/ENVEL-Convertors-Smartphone-Microphone-Simultaneously/dp/B01M0T6PSF

If the OEM realtek driver is not installed it will not "detect" anythiing plugged into the combo port.  Non TRRS headsets require the splitter cable above Green is headphones and RED is mic.

https://dl.dell.com/FOLDER05453354M/7/Intel-Chipset-Device-Software_7FVVD_WIN_10.1.17861.8101_A06_01.EXE

https://dl.dell.com/FOLDER05798156M/1/Realtek-High-Definition-Audio-Driver_GRDHH_WIN_6.0.8777.1_A05.EXE

@zardy 

First, you have to reinstall all the software that was mentioned above.

Second, you have to open the Win 10 Settings screen for Microphone Privacy and make sure the Microphone is turned on:

And after you turn the microphone on and choose which Microsoft apps and desktop software are allowed to use the microphone, you should go to the Win 10 Microphone setup screen and check those settings...

This is OK as far as it goes, but when I installed the Realtek driver and MaxxAudioPro app from https://www.dell.com/support/home/en-us/product-support/product/optiplex-7070-desktop/drivers, the mic still did not work.

It was necessary to open the MaxxAudioPro app, choose "Advanced" on the "Playback" screen and check “Show pop-up dialog when device is connected”. Once I did that, I then got a dialog when I plugged my headset in that let me choose "Headset" which enabled the mic.

@vdavwil - Glad you got it working. Didn't realize the 7070 uses MaxxAudio too.

And I don't understand why that "show popup" box isn't checked by default... 

Wow, thanks for posting!  Worked for me too.

It was necessary to open the MaxxAudioPro app, choose "Advanced" on the "Playback" screen and check “Show pop-up dialog when device is connected”. Once I did that, I then got a dialog when I plugged my headset in that let me choose "Headset" which enabled the mic.