Urgent: Server Refusing Connections After IP Configuration Change, What's Wrong with Subnet Mask?

I'm completely stuck and tearing my hair out. After making what I thought were simple IP configuration changes on my new server, it's refusing all connections.

I can't even ping it internally. I've double-checked the gateway and IP, but I'm starting to think my subnet mask is completely off or I'm missing something fundamental.

Here's what I'm seeing:

ping 192.168.1.100
PING 192.168.1.100 (192.168.1.100) 56(84) bytes of data.
From 192.168.1.101 icmp_seq=1 Destination Host Unreachable
From 192.168.1.101 icmp_seq=2 Destination Host Unreachable

What on earth am I missing? Anyone faced this exact nightmare before?