Not possible to access dashboard

Hi WPMUDEV team,

I hope you are all well 2 days before Xmas !!

I have a connexion problem to the dashboard on my site.
I began to use fixer theme, i'va made some modification to the template.
And now I can't access to the dashboard.
I lost on the connexion screen my logo.
If I give a bad password, I have an error message.
If I give the good one, I stay on the same screen.
I tried to reset the password. The procedure is correct, but the result is the same.

What can I do ?

Thanks for your help and merry Xmas to everybody.