If you can't get Activity monitor to come up.

I would suggest quering the sys views. something simiar to

sys.dm_exec_sessions s

LEFT JOIN sys.dm_exec_connections c

ON s.session_id = c.session_id

LEFT JOIN sys.dm_db_task_space_usage tsu

ON tsu.session_id = s.session_id

LEFT JOIN sys.dm_os_tasks t

ON t.session_id = tsu.session_id AND t.request_id = tsu.request_id

LEFT JOIN sys.dm_exec_requests r ON r.session_id = tsu.session_id AND r.request_id = tsu.request_id

OUTER APPLY sys.dm_exec_sql_text(r.sql_handle) TSQL

This way you can get a TotalPagesAllocated which can help you fiure out the spid that is taking all the server resources. There has lot of times when i cant even bring up activity monitor and use these sys views to see whats going on.

I would recommend you reading the following article. I got this reference from here

http://tsqltips.blogspot.com/2012/06/monitor-current-sql-server-processes.html