php error suddenly

My WP site & plugins are up-to-date and since today I have a strange situation. I can’t enter wp-admin anymore, see for yourself the result:

http://www.bartvandermark.nl/wp-admin

Also my normal homepage has a strange look wheren various items look weird: http://www.bartvandermark.nl/

How can this be repaired and how is this caused?

With kind regards, Bart