To add to Master66's post, if there is SEU cable rather than a mast I would suspect the neutral (grounded conductor) in the meter can. Check the connection in the disconnect if there is one and check the connection in the panel, if all checks out, call the PoCo.
If not there, then one of the other PoCo splices has gone bad.
On the off chance, do make sure that the two circuits in question aren't on a multi-wire (with a loose neutral).
