White blank page when accessing website and Craft login page I'm getting white blank page when trying to access my website and Craft login page. I haven't updated the website for few months. So I assume no files has been changed.

I checked with the hosting provider, and they say everything is okay on their part. 

So I turned on the Dev Mode. And it came back with this error :
Fatal error: Class 'Craft\HttpRequestService' not found in /home/alpw7814/craft/app/framework/YiiBase.php on line 213

What could possibly cause this? Is there anything I need to check?
Thank you in advance.
That error indicates that Craft is missing some files (or they are corrupted) in its craft/app folder.  You can download a fresh copy of Craft from https://craftcms.com and replace your existing craft/app folder with the new one and that should resolve it.