Grub is never automatically reloaded but can be reinstalled. It's fairly
complicated and I've only done it once.

Whether your system will now boot to Linux depends on where you told the
Ubuntu installer to put the grub startup menu, probably on what was the sda
drive but now will be sdb. If Windows is on the first drive Ubuntu won't
boot unless you unplug the windows drive, and make the Ubuntu drive sda
again. That is worse than an inconvenience...

There is a fairly easy to use repair disk that will fix this so you will be
able to choose the operating system from grub as you boot up the computer.
It is available at https://help.ubuntu.com/community/Boot-Repair . It's
saved my bacon more than once!