Hi Sparkly,
What a rubbish thing to happen.
You need to start by looking at your employee handbook and/or your contract of employment. The employee handbook should have details of what constitutes gross misconduct and you will have to see if anything shown there is similar in severity to the implications of your mistake.
If you do not have an employee handbook then look at your contract. If you have neither then you need to ask, politely, for the companies human resources manager to forward you these asap.
Was the company aware of your diabetes and that you were at risk of a hypo - if so you should consider the steps that they and you had taken to reduce the risk to your work and by implication the company. If they were not aware, is it because you did not tell them or because you have not been asked. Most application forms havea mdical history section but I have a feeling they're not allowed to ask this any more.
I am not an expert - though I run my own company and so have to do these things for my own staff. My only concrete piece of advice (other than getting proper advice) is that you write EVERYTHING down - exactly what happened, who said what and when and then keep writing everything down until it is resolved.
good luck
S