THANKS ALL for response and your valued time.
Yes Liath, I had made a website folder backup through FTP and database backup through Cpanel>backup , 2 times in last 1 month. Its on my home desktop PC (before the host provider Cpanel upgrade). I had even tried restoring those backups but in vain with same Error 500 recurring.
This is how I restored the OSClass website :-
1. deleted all files in website folder on server
2. dropped all tables of database and made it empty.
3. copied all the website folder files (backup on my PC) by zipping it and then extracting on server
4. restored database from Cpanel>backup>
(I hope its correct ?)
Yes SmaRTeY , you are fully correct, it seems to be hosting problem, because, just now I had uninstalled OSClass and installed the latest OSClass once again through Cpanel>Softaculous Installer. Installation was success, but the Error 500 persists.
Surely, tomorrow I will be talking with them regarding Apache config.