Is there any way we can get the logs in the timeperiod when the server is down?

With a single box, no, it's the same as with your current Syslog server. If the service is down, logs usually get lost because most Syslog sources can't buffer

is there way so that we can get all the data with single server

No, you will always have the potential of loss. Even with multiple servers, and a load balanced solution, you might run into data loss. Engineering a completely lossless solution is a lot harder than it sounds

thanks for the response. In your opinion what is the ideal solution to collect syslog data using cribl.

Well, I'd still use Cribl. You can use multiple workers plus load balancer to reduce the chance of loss.

thanks.

https://cribl.io/blog/syslog/ is a three part blog on this very topic.