At the moment, I’m trying to write lyrics in English for one of my compositions. But I have second thoughts, if one of my used words exists at all. In any case, it doesn’t appear in a dictionary. The sentence is: “let the sun reshine upon…”. Is it completely wrong to use the word “reshine” instead of “shine again” (as this would not fit the rhythm of this passage), or is this word my own invention?
Hi, I like the word, but it doesn’t exist in English in normal use, although any native would understand it clearly; and in a song, with poetic license, it might not strike an English native’s ear as being too strange.
I totally agree! Poetic license lets you get away with a lot. 
If it fits your song, why not use it. You could also shorten your phrase to “shine 'gain” which would also take out one syllable. 
Thanks for your responses! Meanwhile I found another solution. I have to admit, I really have a romantic bent when it comes to writing lyrics