Can't get in gear - clutch problem?

[klargon]

Howdy!  Got in the car last night and it would not go into gear (meant I had to run to the football instead!).  Can stick it in gear when the engine isn't running.  If I put it in gear before starting, I can't get out of that gear.  I'm assuming this is a clutch problem, but I haven't noticed any indication of a problem developing.  I drove it for about 80 miles the previous night.  I have, however, been intermittently getting a Service ESP warning on the dash for the past week or two.  It's obviously pretty cold at the moment - I'm not sure if this could have triggered/exacerbated an issue.  Any ideas what the issue is likely to be, and if there's anything I can do to get it in a drivable state so I can get it to a garage?

It's a 2010 2.0 CDTI Elite.  About 40k miles.

Thanks in advance for any ideas.