If we are using the computer , suddenly it's not opening , but the switch , router and application server are all pinging fine with its respective IP , then what may be the problem?

If ping successful to the router but internet is not accessible, its mean the problem with gateway, try to reconfigure the gateway. Hope It will work

May be firewall in PC on or some GPO. I'm also not understand your question what is not opening.

Please clarify your question !

What is the thing which did not open ?

Your question isn't clear... What is not opening? However if it's home network or office and like you said everything is pingable then your firewall or antivirus program may be blocking certain ports that the application is running on. So try to disable both and then access. Also rebooting PC isn't a bad idea to refresh tcp stack.

if its taking long time to boot up when connected to e network then the problem is with the AD