The PSU might be either damaged or in protect mode. Try the following steps to troubleshoot:
1. Unplug all the power cables of the PSU.
2. Check if PSU powers up. If it does, reconnect the power cables and make sure they are connected properly and securely.
3. If PSU still does not power on, remove the connector plug with the white silicon paste on it.
4. Use a paperclip to short the green and the black wire.
5. Check if PSU powers up. If it does, the connector plug needs to be replaced, otherwise the PSU is likely damaged.