Guild Wars 2

I switched to a European server to help a friend, and now I can’t switch back the server I was originally on. What gives?

You have to wait until it becomes “un-full”.

Edit — Also this announcement:
https://forum-en.gw2archive.eu/forum/info/news/Free-World-Transfers-Limited-to-Once-Every-7-Days

You can not ever join a Full server. You need to join it when there population is at its lowest point. Like the following Example:

Wait until it is about 2:00 AM during a work week so the server population it at its lowest, then transfer.

If the server is Full, even at its slowest hour of the day, then you will never be able to join it until the population is lowered by less people playing at that time.