Hi,
It also happened to me and it was because I left the default configuration of memory of the JVM (i.e. 64 M), and did multiples import tasks of large zip files. To recover the service, I started modifyng the JAVA_OPTS variable in the ./tomcat/bin/catalina.sh file (e.g. add the line JAVA_OPTS="$JAVA_OPTS -Xmx512m" at the beginning of the file) <however i don't know if it was really necessary to started up alfresco again>, and then i set ./tomcat/temp/catalina.pid to the process id that was assigned to alfresco before it crashed.
Then I could stop Alfresco, and start it again without problems, but now I have to check if the indexes are ok.