can't read serial monitor

I have a problem i tried to read the rpm of my pc fan. I normaly run it on 16v ,to read it i ran it on 12V . It all worked perfect then i wanted to know what the rpm were at 16 v then somthing started to smell (like smoked transistor). Now i cna't read my serial monitor anymore.?
Can someone help me pls.

Thanks
Daneil.K

You need to post a wiring diagram of how you connected it all up. Running 12V or 16v directly into an arduino will fry it. The voltage in has to be no more than the supply voltage, normally this is 5v.