Firstly, I assume this code is on the OnChange property of the drop-down - also the issue here (now that I have my mind around what you are trying to do) is not the checkbox - it is the field name in the table that the checkbox sets to true. I will call it NotShow in the below - just substitute your field name for this.
I also noticed you have said the field name in the drop-down is Accident Name - I may need a bit more clarification on this, however try this code.
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