Uptime in the literal sense is time since boot (uptime shows it next to load average). An unexpected reset means the server rebooted or crashed — worth investigating every time.
In the hosting sense, uptime means service availability: the web server answering, the database accepting connections. The literal number can be perfect while a service is down — which is why per-service watching, not just ping, defines real uptime.
Frequently asked questions
My uptime reset and I didn’t reboot — what happened?
Either the provider migrated/restarted the host, or the kernel crashed/was OOM-pressured into a reset. Check logs around the boot time and ask the provider.
Is very long uptime a badge of honour?
Not anymore — it usually means kernel security updates were never applied. Planned reboots after updates beat a two-year uptime.