Access problem

I’m using Win XP sp2. Just few days no one could access my PC what i shared. But i can access all other pc from my pc.
We can suggest it’s sharing problem but not.
Bcoz,Even no one can able to ping my pc.It show’s “request timed out”.
All the services are running without any issues.And there is no such firewall software except windows security center package.
It was fine when the firewall is on also.sud.. suddenly i lost my connection with out any clue.

Thanks in advance if any one can come forward to help resolve this issue.