My daughter has a 01 blazer 4.3. She drove it to work one day, at end of the day it wouldn't back up, had been workin fine. Can anyone tell me what to look for

 

https://www.700r4transmissionhq.com/4l60e-no-reverse/

 

I had the same thing happen, you probably don't have 2nd gear either. These transmissions have a weak sunshell in them. With the rebuild, you need a rebuild, they should install the heavy duty version. It's better than the one that was installed originally. Be prepared to pay $1500-$2000.