Cannot login in wp-admin

I have been working building up an online shop for a while, but a few months ago I had to stop for personal reasons.
Now I tried to start again but I dont know why, I cannot login in the wp pannel. This is the message that it show:

Notice: Undefined index: loginFacebook in /homepages/12/d347788967/htdocs/tienda_pegatinas/wp-content/plugins/nextend-facebook-connect/nextend-facebook-connect.php on line 118

Warning: Cannot modify header information – headers already sent by (output started at /homepages/12/d347788967/htdocs/tienda_pegatinas/wp-content/plugins/nextend-facebook-connect/nextend-facebook-connect.php:118) in /homepages/12/d347788967/htdocs/tienda_pegatinas/wp-includes/pluggable.php on line 678

Warning: Cannot modify header information – headers already sent by (output started at /homepages/12/d347788967/htdocs/tienda_pegatinas/wp-content/plugins/nextend-facebook-connect/nextend-facebook-connect.php:118) in /homepages/12/d347788967/htdocs/tienda_pegatinas/wp-includes/pluggable.php on line 679

Warning: Cannot modify header information – headers already sent by (output started at /homepages/12/d347788967/htdocs/tienda_pegatinas/wp-content/plugins/nextend-facebook-connect/nextend-facebook-connect.php:118) in /homepages/12/d347788967/htdocs/tienda_pegatinas/wp-includes/pluggable.php on line 680

Warning: Cannot modify header information – headers already sent by (output started at /homepages/12/d347788967/htdocs/tienda_pegatinas/wp-content/plugins/nextend-facebook-connect/nextend-facebook-connect.php:118) in /homepages/12/d347788967/htdocs/tienda_pegatinas/wp-includes/pluggable.php on line 896

Can some of you help me please?

thank you very much!