This problem also happened some time ago and after a period of time it "magically" worked again.... But it has now been about a month that it will not work again.
you have installed CMSimple SE. That's a very old CMSimple Fork, and it's not under development since many years.
I fear, you have to begin from the scratch. How you can do that, you will find here:
http://www.cmsimple.org/doku/en/?Update ... d_versions
With a little bit luck you can use your old content, if the converting to utf-8 is successfull.
I didn't find any plugins on your site, so you will have some problems less