My Eshop Crashes since the Last 5 days

[crislia]

Hello, 
I am really disappointed with the costumer and support service of presta. 
I am facing a major problem with my shop and I have tried every form of communication with the "Specialists" but in vain.
Since the communication with the Technical Support Team is impossible, I ask for your advice here.

My eshop is on the cloud of prestashop, I didn't make any change or any new installation and suddenly it started to crash.

The problem is at the frontoffice only and it starts when you are trying to make a purchase, at the checkout appears a message "HTTP ERROR 500". If you refresh the page the same message appears.
If you try after that to visit again the homepage you get the same message "HTTP ERROR 500".

Where do you think is the problem? 

Thanks in advance.

Edited October 20, 2016 by crislia (see edit history)

[shokinro]

were you able to enable error displaying by change following _PS_MODE_DEV_ from false to true in file /config/defines.inc.php, and then try again there should be more specific error message displayed. from the error message you may be able to figure out what's going wrong.