Can existing key pair be used if I restart the ec2 instance(stop and start) in aws or do we need to create new key pair or launch another instance?

I am trying to connect to linux ec2 instance feom windows pc. Firewall is open on port 22. But I am getting Network Timeout while connecting through putty. I have tried to debug, but everything seems fine. I am able to connect using browser based ssh instance but not using putty or standalone ssh client. So my only doubt is if I had to create a new key pair because I once restarted the ec2 instance.