Why isn't my email sending?

Since yesterday, I've been trying to send a really important email via the Gmail app, as usual, but it keeps getting stuck in the queue and won't send. I've tried using a different Wi-Fi connection, using mobile data, and restarting the entire phone. I can't think of anything else.

Shortly afterwards, "Email is being synchronized" always appears in the bar at the top and stays there.

What's wrong? What can I do? This is really important

Thank you very much in advance and please be kind even if the answer is obvious. I'm simply lost when it comes to technology…