Without trying to guess too much, the best shot at solving your issue is to try to figure out a few things:
1. A consistent repro. If you can consistently connect and then consistently not connect we might be able to find a change that could be the culprit
2. Provide exact repro steps so that the community can attempt from our side.
3. Do you have some system details that you can share (MsConfig, nature of your PowerPivot connection)
4. Help us narrow down the list of suspects. Do you have multiple programs attempting to access Powerpivot at once? Do you have admin privileges to your computer and have you attempted to open all programs as an admin? Try turning off your anti-virus; kind of a shot in the dark, but anti-virus can sometimes interfere with your connection--even if it's local.