Unable to view pages anymore

Hello
I developed a website that has been up since few months back. It had been running well (at least I know two weeks ago) until today when I noticed that I no longer could access a page from the menu. Each time I click a menu it displayed an error page.
So I checked each page from the dashboard, the codes were there but when I clicked view page it displayed an error message.
Here is the website…
https://www.mehsa.org
Do you know what went wrong?
Myo