There are two kinds of crashes in a client/server environment.

Client crash
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This is when a single client goes down. In this case, the server will not be affected, nor will the other clients. That user will lose any unsaved changes to whatever record they were working on at the time. To recover, they just to restart their client.

Server crash
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This is when the server itself goes down. In this case, all users will lose any unsaved changes to whatever record they were working on at the time. When they try to do something, they will receive a message that they have lost communication with the server. To recover, you will need to unlock the database and restart the server. Then your clients can reconnect.