Hi,
It would certainly be helpful to at least know the Make/Model of your computer. And is it a Desktop PC or Laptop? Did this computer come with W10 pre-installed from the factory or did you upgrade it from an earlier Windows version such as W7/W8x?? We can advise you better with that much information. Windows Defender updates use the same mechanism as Windows Update (WU) for the most part.
When those don't work that's usually because of one of these problems:
1.) Your computer has been attacked by viruses/malware. There are many new viruses that attack and disable specifically your computer's ability to perform WU as well as Windows Defender. Hackers are all aware of these 2 tools and how to disable them.
2.) Your computer has suffered Windows corruption in the registry, possibly due to removal of viruses/malware found, or installing programs that are poorly written and when you run the uninstaller for the program, Windows stops working correctly and will throw various error messages up on the screen or produce BSODs, freezes, or hangs.
3.) If you had upgraded your computer to W10 from an earlier version of Windows as mentioned, W7/W8x, it's very likely that your computer hard drive is 3 years old or older. If that's the case, it's
VERY likely that your hard drive is failing or has failed and is actually producing that error message. We've seen that error message and many similar ones when hard drives begin to fail. You should also be aware that hard drives in modern computers are only designed to work for 3 years and then begin to fail!!
And only 2 years in laptops!
.
You are now at the point where you might need professional help to resolve this problem. It all depends on the level of your computer repair skills. You can pay a Tech to repair this for you or you can attempt it yourself.
If you wish to try yourself you must first backup all of your personal data (from Library folders) to external media before attempting any Hardware Testing or Software repairs IN ORDER TO AVOID IRRETRIEVABLE DATA LOSS!!! Take a look the excellent and free TROUBLESHOOTING GUIDE I wrote that will give you step-by-step instructions for troubleshooting your W10 computer hardware, as well as repairing faulty windows, due to corruption or virus/malware attack. The GUIDE is available here: Windows 10 - Unclickable Task Bar This GUIDE is quite long, so I suggest you print it out on another working computer, so you can have hardcopy in front of the computer you are attempting to fix. This is especially important if you have to perform a W10 Reset or Reinstallation from Factory RECOVERY MEDIA.
If you are patient, and follow the instructions sequentially and don't skip any steps, this process should produce a fix 97% of the time. Of course, there are a few exceptions to this, and if you get stuck along the way, post any questions that you have back to this thread. Expert computer techs are here to help you 24x7x365.
Best of luck,
<<<<BIGBEARJEDI>>>> 
