well, this happens in my "test" database, max memory utilization results in this...(in my case, so i restart the box)
you use performance related commands like
also check RAM utilization (a must)
and find out where max memory is being used, if that process is like not responding kill it & check if db is connecting.
if its production I would think 1000 times before rebooting...
yes as oscrub mentioned check "sqlplus -prelim / as sysdba".