I managed to unstuck it with frantically pressing alt-. 

and where was it stuck?

One of my live objects didn't stop last time I saved and closed Squeak. It was stuck in some loop when it came back on.
 
On Windows I have no idea but I assume you're not on Windows cuz we don't have the 64-bit JIT VM working there yet.


Is this not a 64 bit VM on Windows? Using it right now...

I guess it's only a StackInterpreter VM.  If you use the About Squeak tool what's the VM version info you get?

Virtual Machine
---------------
E:\Programmes\Squeak5.1-16549-64bit-201608180858-Windows\Squeak.exe
Croquet Closure Stack VM [StackInterpreterPrimitives VMMaker.oscogLLP64-nice.1932]
Win32 built on Aug 19 2016 21:50:35 CEST Compiler: 4.2.1 Compatible Clang 3.7.1 (tags/RELEASE_371/final)
platform sources revision VM: 201608180858-LLP64_v2 nicolas@PC-CEC:C:/Users/nicolas/Documents/opensmalltalk-vm $ Date: Thu Aug 18 10:58:48 2016 +0200 $ Plugins: 201608180858-LLP64_v2 nicolas@PC-CEC:C:/Users/nicolas/Documents/opensmalltalk-vm $
StackInterpreter VMMaker.oscogLLP64-nice.1932 uuid: 642ecc37-5300-3643-8d95-5acb4704c924 Aug 19 2016

Seems you are right.

Ian