500 Error when trying to access a .php file

[globay]

I am having quite a severe problem on one of my servers. I use a lamp dedicated managed server and for some reason, I am getting 500 errors when trying to access .php files (even if they are just text files) for about 12 hours now. I did not change anything during the last days.
I won't be able to reach server support until tomorrow, so does anybody know what the reason could be, and if there is a way to fix it?
Thanks,
globay

[lemat]

do you have any logs?

[globay]

yes, logs show 500 errors for all files accessed, except for the 500 error page files. (I still use the predefined 500 error page that my hosting company built)