The solution depends on what program caused the crash.
1. Magic Contest crashed
This is no big problem as Bridgemate will continue to import results from the Bridgemate server. Do the following:
Close the error window. Restart Magic Contest and open the existing event in Contest Explorer. There will be a message about a lock file. Choose to delete it. Go to Contest menu > Enter results. Resume Bridgemate import by activating the Bridgemate checkbox. All will be as if the crash never occurred.
2. Bridgemate Control Software (BCS) crashed
Now there is no chance for Magic Contest to import the results as they are (temporarily) stuck in the Bridgemate server. The fact that the players can continue entering results is completely according to design. Solve it the following way:
Close the error window. Ensure that BCS is completely shut down. Then click Contest menu > Bridgemate > Open Bridgemate Control. This will not only open BCS, but also include information about what Bridgemate database to use. Now you can resume the Bridgemate import in Magic Contest through Contest menu > Enter results and the Bridgemate checkbox.
3. Windows completely crashed
Restart Windows. Open Magic Contest and open the contest from Contest Explorer. The resume according to #2 above.