Server suddenly became slow

[DrByte]

I didn't change the code or installed modules in last 2 weeks when website suddenly became slow .

Okay, so by process of elimination:
- "you" didn't change any code
- nobody "here" changed any of the code
- Zen Cart authors didn't change your code, because they don't have any access to your site/server/software/store
What other players are involved here?
Oh: the server. Run by the hosting company.
Bingo. There's the problem: something's wrong with the server.

Maybe the server's overloaded?
Maybe the database engine is damaged?
Maybe the hosting company's administrator changed database-server settings? Maybe they upgraded the version and broke something or corrupted something or didn't realize that their automatic upgrade also changed configuration settings that are now causing bottlenecks?

Only they can speak for what's happening behind closed doors.

Why pages like product details are not slow ? ( Parse Time: 2.382 - Number of Queries: 453 - Query Time: 1.81040376318 )
Why querying tables ( using phpmyadmin ) is not slow ?

I suppose it's possible that your database tables might need a "repair" run on them to resync/rebuild indexes and such.
You can use phpMyAdmin to select tables and choose Repair Table, at least from the Operations tab. Some hosting companies offer a "Repair Database" button which does this "repair table" on all tables at once, which is a lot more convenient.