Please could someone tell me why I should be really worried about Brexit with no deal

Well with no disrespect intended Gonegirl it depends how you and yours will be effected.

Here in the North east we rely heavily on big industry to provide jobs for 1000's of people plus those first, second and third tier suppliers (such as ourselves) .

If it all goes horribly pear shaped Nissan could pull out of the NE and the UK altogether leaving absolute devastation in their wake. Not forgetting that Renault have around a 55% stake, effectively put them in control . I can imagine that Nissan/Renault moving to France would be just what the doctor ordered for Macron et al.

That was just one example, there are plenty more, but as I said it depends on your own circumstances how you will be effected.

Nobody knows - that is the problem.