2000 Blazer not shifting to any gears
 

Blazer has been shifting fine. Parked it last night, it was about 10 below 0, this morning nothing happens when you shift into any gear. I looked under the car and can see that the linkage is moving when the shift lever is moved. I don't know if this is related but I just replaced the rear disc brakes/rotor/calipers etc.... I test drove after the replacement and everything appeared to be OK. I have not checked the fluid level yet as this happened this morning and I needed to take another rig to work. Is there any way this could be caused by the cold?

really doubt the cold did it. One thing that has been posted a lot is the tran filter falling off. That would cause it. But there is a lot of other things possible also

Get it good and warm to normal operating temp then check fluid first. as has been said if that is ok it is possible the pick up filter and tube may have fallen off in the pan. next after that is shift solenoids and then pump pressure or lack there of.