Check 1: the bot token
If clicking Start Bot doesn't bring it online in Discord, the token in your project's settings not matching what Discord currently expects is the single most common cause. Tokens can be regenerated from the Discord Developer Portal (this immediately invalidates the old one), if that happened for any reason, paste the new token into your project's settings and try starting the bot again.
Check 2: Privileged Gateway Intents
If your bot's commands or events rely on data like server member lists or message content, the corresponding Privileged Gateway Intent needs to be explicitly enabled for your application in the Developer Portal, Discord disables these by default for new applications as an anti-abuse measure. Missing intents can cause a bot to fail to connect at all, not just misbehave.
Still not working?
Reach out with your bot's application name (not the token itself, never share that) and roughly when you last saw it working, that narrows down what changed.