A matrix of that size and class (single complex) will take up about 127 GB of RAM, which is pretty close to half your total RAM, so if you have any operations in your code where MATLAB is going to make a temporary copy of that matrix, you're very likely to run out of memory. (The extended matrix size would take about 9000 GB, in which case you're guaranteed to run out of memory.) My guess would be that it was indeed a memory issue, and it might be a good idea to figure out an alternative method whereby the entire matrix doesn't need to be loaded into RAM at one time (and such a method will be required if you move to the extended matrix size).
However, MATLAB doesn't usually shutdown suddenly when it runs out of memory; it usually throws an Out Of Memory error, so I could be wrong. I'm mostly speculating based on the specified matrix size and amount of RAM.
