The padlock means that the bootloader is unlocked. It has nothing to do with root. I don't think anything changes if you re-lock it either.
If you're running stock and if you're not rooted anymore (how did this happen anyway?) just use the factory reset on your tablet, it's the same thing.
Use the superuser app (unless that one is gone? in this case just reinstall it from the play store) to unroot the device, and factory reset it then.
Are you using Windows 8? There are issues with fastboot/flashmode on W8, there is no fix, you have to use W7 or something else
Nokia Lumia 925 16GB
Nexus 7 32GB, 4.3 stock, rooted
Sony Ericsson Arc S, 4.0 .587, rooted
Dropbox for you: http://db.tt/Qk80xRg
500MB extra for both of us.