IF you only have access to your computer as a "Guest", then you're not going to be able to get much of anything accomplished.
IF you cannot remember the password for the account that you were receiving the credentials prompt for, then you are likely out of luck.
You are going to need, at least, the installation media that matches your current installation of Windows 10
https://www.microsoft.com/en-us/sof...459594)(TnL5HPStwNw-WaTVja5C7z2Vs7q82apCPg)()
Choose Windows 10, the first item in the first drop down (not single language at the bottom)
* see note at bottom.
Next choose your language and your bit version (32 or 64 bit to match your system architecture or in the case of an upgrade, to match your currently installed version of Windows).
That will provide an ISO that will boot, upgrade, repair (or clean install) either or both Windows 10 Pro and Home.
*NOTE: In some cases, we've learned that some people have the "Single Language" version installed. In which case you would need that download. It's important that you match the ISO version that you download with your installation.
Once you have downloaded the ISO you can use ImgBurn to burn it to a DVD
http://imgburn.com/index.php?act=download
OR
Rufus to burn it to a USB ThumbDrive
http://rufus.akeo.ie/
That you can use to boot your computer
When you've managed to boot from the installation media you need to click the Repair Your PC link on the second page after the Choose a Language and Keyboard layout page.
That should get you to the Advanced Troubleshooting options, where you should have some options you can try.
System Restore, reverting back to a previous build, or even Startup Settings, which allows you to boot into Safe Mode, with Networking.
What you will more than likely need to do is rescue your computer by enabling the normally hidden / disabled Administrator account and use that to possibly fix the password problem on your local user account.
Again, boot from the installation media and
Follow the steps outlined here
http://pureinfotech.com/2015/11/20/how-reset-windows-10-forgotten-password-regain-access-computer/
It's a pain, but it works, I know, I've done it.
Once you've enabled the hidden administrator account you should be able to at least backup any critical data and hopefully resolve the other account password issues.
IF nothing else by creating a new user account and then copying the contents of your profile folders from the old account to the new.