Today was the first time that my Lenovo tablet PC lost track of its pen (this was fairly common on my previous tablet, a Toshiba Portege).
When this first started happening to me, I used to restart the tablet in order to get it to rediscover its pen. Then I discovered that it was sufficient to close the laptop and reopen it (getting it to sleep and then wake up).
Today I had actually just finished the class when it happened. Now the “autorotate” settings on my tablet are such that it has a rethink if you change from tablet to laptop mode. That turns out to be sufficient for it to find its pen again! So, if this happens in the middle of a class, I will see if I can get away with simply twisting the screen and twisting it back.