Windows Defender in Windows 8.1 Will Not Turn On

I need to turn on Windows Defender for 8.1 please help me?

could you be more specific? ...

control panel > ? (what area is the action center in) > windows defender

I am not able to turn on my windows defender 

1. My windows 8 pro did not come with antivirus software ... I built this system myself ... I downloaded and installed windows defender ... it worked great until I updated to windows 8.1 ... 

I just tried turning it on and it says "This app has been turned off and is not monitoring your computer. If you are using another app to check for malicious or unwanted software, use action center to check that apps status".

Yes I also tried it in admin mode. still get the same message ... Nothing allows me to turn it on or off ... not even in action center ...

Edit: It should also be noted that the methods that people keep posting about how to click windows defender and turn it on do not work ... the pop up that you see in the picture above is all that ever comes up ... no matter where or how I access windows defender ... 

This issue only existed after updating to 8.1 ... 

Also in reference to another post ... the firewall protection is not windows defender ... action center says it is on and working ... even though windows defender is not on and not working ...

Final Edit: This should help allot of people ...

I noticed a post from another site that mentioned typing "security" in the windows search charm ... then clicking on check "security" ... this option did not work for me ... there appears to not be anything about security (in reference to windows defender) on this computer ... 

I went and installed bitdefender ... did a full system scan ... It found none of the viruses or spyware ... (the new version of it is very poor) ... I then unistalled it and it prompted a computer restart so I did

Bam! the action center notified me of an issue ... I checked the issue and it told me my computer was not protected and windows defender was turned off. then asked if I wanted to turn windows defender on ... Uh ... Yeah ... now it works and allows me to make changes to it ... 

so If you are having problems with security I would recommend installing something like BitDefender run a quick scan or whatever you can get to work ... turn off your computer then turn it back on ... (not a restart) ... then uninstall it using the uninstall software feature in the control panel ... (this should automatically use that programs uninstall function) ... whether or not it prompts for a restart you should then restart your computer ... then follow the action center prompting ... hope this works for you ...

I had same problem. I had a trial antivirus program that was blocking windows defender, don't know why Microsoft would allow this, but after I removed the program I was able to turn on windows defender. Instructions for turning on Windows defender is on the community web site.

Thank you very much Method-1 worked for me. I was trying it from 2 days to turn it on.

I am unable to launch windows defender it has been turned off by something. I don't have any other virus protection. Is there any way to turn it back on. i have gone to the start menu and tried to run it as an administrator i get an error message that something else is protecting the computer but its nothing there but window defender in the action center.  I am unable to get it to work. 

Thanks in advance