Site-to-site VPN tunnel is up, but network connectivity has stopped

Hi @Sree ,

First of all, please check whether there are high CPU/Memory usage issue on both FGT devices or not.

If no, please run the following command for a Ping traffic flow:

diag sniffer packet any 'icmp and host x.x.x.x' 4   // x.x.x.x is the IP you want to Ping

If you see the abnormal sniffer packet capture on which FGT, run the debug flow commands on that FGT:

https://docs.fortinet.com/document/fortigate/7.6.1/administration-guide/54688/debugging-the-packet-flow

Please apply the x.x.x.x IP as the "addr" filter for the debug flow commands.

Also use the 'diag debug flow' command. The article Troubleshooting Tip: First steps to troubleshoot connectivity problems to or through a FortiGate with sniffer, debug flow, session list, routing table describe how to troubleshoot connectivity between networks. 

Try to isolate the issue, once you do not know what is causing it. By the way, which firmware version are you using?