I managed to unstuck it with frantically pressing alt-. 
 
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.

http://files.squeak.org/5.1/Squeak5.1-16549-64bit/Squeak5.1-16549-64bit-201608180858-Windows.zip

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

Ian