Stored procedure hangs seemingly without explanation

we have a stored procedure that ran fine until 10 minutes ago and then it just hangs after you call it.

Observations:

Copying the code into a query window yields the query result in 1 second
SP takes > 2.5 minutes until I cancel it
Activity Monitor shows it's not being blocked by anything, it's just doing a SELECT.
Running sp_recompile on the SP doesn't help
Dropping and recreating the SP doesn't help
Setting LOCK_TIMEOUT to 1 second does not help

What else can be going on?

UPDATE: I'm guessing it had to do with parameter sniffing. I used Adam Machanic's routine to find out which subquery was hanging. I found things wrong with the query plan thanks to the hint by Martin Smith. I learned about EXEC ... WITH RECOMPILE, OPTION(RECOMPILE) for subqueries within the SP, and OPTION (OPTIMIZE FOR (@parameter = 1)) in order to attack parameter sniffing. I still don't know what was wrong in this particular case but I came out of this battle seasoned and much better armed. I know what to do next time. So here's the points!

Solution

Run Adam Machanic's excellent sp_WhoIsActive stored proc while your query is running. It'll give you the wait information - meaning, what the stored proc is waiting on - plus things like the execution plan:

http://www.brentozar.com/archive/2010/09/sql-server-dba-scripts-how-to-find-slow-sql-server-queries/

If you want the outer command (like a calling stored procedure's full text), use the @get_outer_command = 1 parameter as well.