If you have linked several accounts within the app, for example because you have several children at different schools, and one of them is no longer linked, you can add the account again as follows:

First click in Sdui on the menu in the upper right corner (three lines) and then on "Switch account". Here you can see which account you are currently logged in with and which other accounts may be linked. If an account is missing, click Add Account, then click Add Parent or Teacher Account. In the next step, select the school of the account that you want to reconnect and log in with your login data for this account.
If you have forgotten it, you can go to "Forgot your password?" request new log-in data.

After you have successfully linked the account, the missing account should also appear again in the pink tab on the right edge of the screen.