snow leopard completely blocked

Hello everybody,
I am now facing to a problem i thought i’d never had using Mac: my computer OS is completely out of service.

Lately my mac started to block quite often, and yesterday it definitively stopped working: when I lit on my computer, I can log in but then everything is blocked.
I only see my desktop, no icons or else, and I can do nothing, not even swith off the coputer without using the manual button.

I assume I have to erase everything and to install again the OS but since I can do nothing with my computer, I wonder how I could do that.

Anyone would like to explain me, please?
Thank you very much

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