I have seen several topics regarding this situation, but none of them truly address my issue (that, or there are no answers that solve the issue.)
Upon booting into Ubuntu 12.04.1 x64 my mouse wired USB mouse (Steelseries Sensei if that makes any difference) either doesn't work or stops working immediately after the grub menu, prior to the login screen appearing. The cursor is present, but obviously not reactive to any mouse movement; even the LED on my mouse turns off (or fails to turn on.)
The only way I've found to activate the mouse is to restart the computer, and even then it's 50/50 at best as to whether or not the mouse will be powered. This issue has never happened in Windows 7, so the mouse is operating properly on the hardware and connection side.
Forgive my ignorance of Ubuntu (and Linux in general), but I was hopeful that someone would have some suggestions, or could at least instruct me on what to check in order to provide any further information for diagnoses.