said by DxViperMan :

hmm, your signal levels changed that much in one night? as a tech, the 1sy thing i would check would be for 1 of 2 things.
1. did you make ANY changes to any cable wiring in the house or outside in the box, like maybe adding a splitter? changing the way it was setup the day before?
2. I would look outside in the box or around the house to see if your neighbor came over in the middle of the night and put a splitter on your line to get cable at his house by stealing it from you.

cable signals don't just change without a reason. there are no moving parts so something had to change to make levels change much more than a few db. it is possible that is could be a mainline issue, but unlikely or everyone in your area would have the same issue and with those levels on the downstream, some modems would never get a connection at all or at best, intermittent