Andi Saitenhieb

You're welcome.

By default, on the Tools Panel Artistic Text & Frame text share the same button position. This is indicated by the (tiny!!!) disclosure triangle at the bottom right of the button. To switch between them either click on the triangle or click & hold on the button so a menu pops up where you can choose which one you want to use. This is "sticky," meaning that once you make a choice it will remain selected until you change it.
You can also customize the Tools panel to show both text tools individually, as explained in the Tools panel link above.

Edit > Undo should fix it, or moving backward in the History panel.
As to how you did it, possibly you pressed Ctrl+Enter (Windows) or Cmd+Return (Mac) while the text (or frame, possibly) was selected.

If using the Undo is no longer a possibility then there’s no way to turn the curves back into text because the software has no way of knowing which fonts (can be multiple fonts used in one frame) were used to create the text in the first place. A curve is just a curve.
The History Panel, as with most other panels, can be shown via the Window menu, e.g. menu “Window → History”.

You're welcome.
There may be some tools out there on the web that can, up-to-a-point, convert curves back to text.
If you really need to convert the curves back to text, and finding out how to do so would take less time than typing the text back in yourself, then it might be worth looking for them.

Go into shortcuts and remove the shortcut for Convert to Curves
Ctrl+Enter (Windows) or Cmd+Return (Mac)

On recent MacOS versions:
export the text as bitmap image or make a screenshot QuickLook the image file in Finder click the OCR (optical character recognition) button in the bottom-right corner of the QuickLook preview window copy the recognized text as plain text, then you can format it again

The Affinity applications have no keyboard shortcut search facility, although one has been requested a few times, .e.g:
https://forum.affinity.serif.com/index.php?/topic/194638-search-engine-in-keyboard-shortcuts/
The one you are looking for, in case anyone else wants to know, can be found in <application>/Layer/Convert to Curves, see attached image.

I didn't know about these two different kinds of text, and I was using Artistic Text because it was chosen automatically.
I just needed to use Frame Text. 😊
Thank you!

Thank you for your reply, Walt!
I know about the undo command, but that's not available anymore once the program was closed and restarted.
My idea was that if it's possible to turn the Frame Text into curves, there must be a way to turn the curves back into a frame text ...
Another question: Where can I find that history panel that you mentioned?
In my Affinity Photo 2.3.1 there is only undo and redo under the edit tab, but no history.

If you are using Artistic Text tool, just click on Artistic text box and type additional text, the box will expand to accommodate the additional text.
Using the Text Frame tool allows you to expand the frame without affecting the text. 

My first guess is that you could be using the wrong type of text layer for your needs.
Have you tried using a Frame Text layer instead of an Artistic Text layer?

A further suggestion - convert the image to greyscale, adjust brightness and contrast to give a 'punchy' black and white image, convert back to RGB, then apply a Gradient Map.
Don't use the "Lighter Colour" and "Darker Colour" blend modes. Use a single Gradient Map, with the blend mode set to Normal, with your darker colour set to the left-hand end and your lighter colour set to the right-hand end.
I've attached an .afphoto file with the history saved and using your two colours - hope it's close to what you want.

 
ADD: To explain a little further - the Gradient Map adjustment maps the left-hand blob (your brown) to the darkest (black) pixels in the original, and the right-hand blob (your beige) maps to the lightest (white) pixels. Pixels with intermediate tones (greys) map to the intermediate tones in the gradient map. That's why I would suggest converting to greyscale and adjusting brightness and contrast - the more contrast in the original, the better the sepia-toned image will appear. If there are no blacks in your original, there will be no dark browns in the sepia-toned version. Likewise with whites.
 
brugge_sepia.afphoto

@Andi Saitenhieb You may want to try using only one Gradient Map layer, as shown below - 
 

Anything like this? The gradient map is in the .afphoto file attached

SacreCoeurGradMap.afphoto

Thank you so much, Joachim!
I already found that option, set it as standard for the first two RGBA things, didn't work.
Now I changed it for all 6 and there it is, my own color palette. 😎
Thanks again,
Andi