Good news - that's what happens, and it's nothing that can't be solved!
Neutral news - you will need to contact the employer to get help with that.
A withdrawn application is permanently withdrawn, so you can't undo a withdrawal. If the application deadline hasn't passed yet, you can instead reapply for the position. However, BEFORE you can do that, the employer needs to completely delete your application from their end. If they don't do this, you won't be able to submit a new application because the system detects that you've already applied for the position.
Once you've spoken to the employer and they've deleted your application, you need to wait until the next day to submit your application again. This is because your application isn't completely deleted from the server until midnight.
If the application deadline has passed, or is about to pass, so you can't submit a new application the next day, the employer has other ways to receive your application. They can, for example, receive it outside the system and manually enter it, or they can send you a special link that can be used after the application deadline has passed.
Check with the employer what procedures they have in those cases!
And look, there were no bad news π