This error pretty much means that your connection has clutched its chest and died. For some reason, your client machine and the database server are acting like an old married couple and have stopped talking to each other. That reason could be one of a rather long list: has your server crashed? has someone pulled out your network cable? was your process killed at the O/S level? is your Windows log full? or maybe there is an Oracle internal error?
My advice is this: do not overlook the obvious. This error is sometimes caused by the simplest of things. If, however, it is caused by an Oracle internal error, look to your alert log for further information.