OSI model layers (bottom-up): Physical, Data Link, Network, Transport, Session, Presentation, Application. In practice you care about L1 (cables), L2 (MAC (Media Access Control)/switching), L3 (IP/routing), L4 (TCP/UDP (User Datagram Protocol)), L7 (apps).
'Can't reach the server' could be: cable unplugged (L1), wrong VLAN (L2), no route or wrong gateway (L3), firewall dropping port (L4), or app crashed (L7). Knowing which layer narrows the fix.
TCP is reliable and ordered (web, email, SSH (Secure Shell)). UDP is fast and lossy (DNS (Domain Name System) lookups, VoIP audio).