I accidently altered my partition table. Somehow I wiped it out completely and changed the disk label. Before I rebooted I managed to rebuild it the way it originally was using the original structure and label however I need to re-install a new bootloader for the new MBR. I have Ubuntu 14.04TLS running inside VirtualBox on a Windows 7 machine. Is there a way to do this without rebooting?
Well, this sounds much like an exclusively Windows issue. I strongly doubt that this has *anything* to do with the virtual Ubuntu installation, because you can't alter the physical harddisk from the VM, afaik.
So let me sum that up: You messed it up, you repaired it, you rebooted
Obviously both is working, bootloader and MBR, otherwise you wouldn't be able to (re)boot.
As a last point, you might find following link interesting:
How to Manually Repair Windows 7 Boot Loader Problems