U must have forgot to clip a connection back or bumped something loose.
first check to see if your alt is charging, if not check your conections if they are all good then check your fuses, I have seen where a alt will short out and blow the fuse, it would be in the fuse box under the hood. If all this checks out return the alt and install a new one
Check what code it's giving you to narrow it down
That is true, I was thinking it was the battery light that was on
Looking for a Used F-150 in your area?
CarGurus has 203,179 nationwide F-150 listings starting at $1,273.
Search Ford F-150 Questions
Ford F-150 Experts