My 2000 chevy s10 pickup doesn't want to shift

I had the same problem and it was the spped sensor!

Check your master and slave, If your slave is not fully depressing the pressure plate it would be very hard to shift.

Low fluid would not cause this. However there is a bolt on the side of the trans that you remove and put your finger in to check the level. If you put your pinky in and no fluid comes out it is low.

The slave often goes on the chevy's. It is internal, you have to pull the trans to replace it.

Also a pressure plate that is collapsing can cause the same problem. You are pushing all the way down, but it is not releasing.
Hope this helps.