interestingly enough i had this issue come up last night when i got home from work... it's never happened before. The red light on the mouse was gone, i was hoping the computer hadn't frozen or something..the keyboard worked fine..pressed a few buttons to wake it up..then the mouse was recognized and it worked... Try going into your Device Manager and selecting properties for your mouse..then in Power Management enable the box Allow this device to wake the computer. That should solve it. The keyboard has it enabled so it makes sense it would keep your mouse from sleeping
Thanks for posting the question, i was able to solve it for my system and figure out it wasn't a fluke kinda thing.