In that situation, I would start checking the prices of the low cost carriers.

I once had an AF ZED from SFO to CDG and the flight was full but VS accepted our tickets for their LHR flight. This was a last minute issue so no time to get a ZED for London to Paris. We did not have an office in Europe.
Our return ticket was from CDG to IAH so we really did not know what we would do. We checked into flights and they were really costly. Someone told us to check the Eurostar, which we did and we ended up getting a one way to Paris for about $100.
Not too bad and we go to ride through the Chunnel...