Log file(s) is located in /workspace/.metadata/ directory. If you want to know the direct location check Configuration Details dialog (Help > About Eclipse SDK > Configuration Details) and look for osgi.logfile entry i.e.:
In the same directory you may find older log files, named .bak_X.log (where X is a number of log file). The older files can be used if the problem occurred in the past.
Sending us the log file(s) please specify the estimate time the problem occurred. This will help us to faster locate appropriate log entries.