Internal engine bang

Strangest thing just happened when leaving work today, driving up a long not to steep hill out of work, there was what sounded like an enormous bang, internal. immediate

loss of power that at first i thought it was seizing but no.  the lights on dash all flashed and the temp went right up and down again. EM light came on immediately and stayed on, the van kept running out of gear but was juddery for a few seconds. Drove it a little further up the hill in and stopped for a look under the bonnet, no issues or leaks and drove home normally, although carefully. Got home and turned engine off for at least 20 seconds and it restarted, no issues apart from EM light on.

Strangest sound it was. like a big thump inside 

Any help appreciated