Since the morning today, l had been unable to access my feddit.online account. It's only in the evening that things are slowly coming back to normal.

Did such a thing happen primarily due to a traffic jam in the fediverse ??

Instances have downtime from time to time, some moreso than others. Per fediverse.observer, feddit.online seems to have a lower uptime rate than other Piefed instances.

I think this can only happen on larger instances that don't have the infrastructure to scale for spikes of high demand. This isn't a flaw of the protocol; you'd have to speak to the people who host your instance about the issues it has.

To be fair, if anything on the internet has too much traffic, it becomes unavailable. That strategy is at the heart of (D)DOS attacks.