Windows 8.1 Update Deleted my ADMIN Account

Here's the story:
I had an Acer Windows 8.
I updated to Windows 8.1.
Tried to download a game(Minecraft).
UAC popped up(With only a NO option).
The YES was greyed out.
I went to User Accounts, but I'm the only user there(Guest account).
I need my admin account back!
I can't open anything as admin(including CMD).
I can't do anything that needs administrator privileges.
Is there a way to fix this?
All I have is one computer, and a 32GB flash drive(I don't know how to use it).
Please teach me how to fix this.
I also don't want to lose any files, apps, settings, or anything like that.
I just want my admin account back.
I can't even enable the built in admin account(I don't know why).

Thanks so much.