A server that will not start, crashes on load, shows online but rejects players, or dies part way through startup.
Almost every crash is answered by the log, so start there rather than with a symptom search. Finding the crash report or logs folder takes a moment, and sending it to us is the quickest route to an answer if you would rather not read it yourself.
If the server will not start at all, the usual causes are a mod or plugin that does not match the server version, a bad startup flag, or a file left behind by a previous install. This guide walks the whole sequence, and this one narrows down which mod is responsible when the log points at a conflict rather than a single culprit.
Most of these are configuration rather than hardware. If a crash only appears under load, it is worth reading about our Minecraft hosting tiers and what each one allocates.