UptimeRobot (Free)
Register at uptimerobot.com. Free plan: 50 monitors, 5-minute interval, email/Telegram alerts.
Telegram Alerts from VPS
HTTP_CODE=$(curl -s -o /dev/null -w "%{http_code}" "https://yoursite.com")
[ "$HTTP_CODE" != "200" ] && curl -s "https://api.telegram.org/bot$TOKEN/sendMessage" -d "chat_id=$ID" -d "text=Site down! Code: $HTTP_CODE"