High Server Load on VPS. Need Help!

I have my WordPress site running on VPS.
From past few days, my CPU getting high suddenly.

Here is the link of my site.

https://www.hotmug.org/

I was running my site on that VPS for 3 months without any problem but now this problem occurs.

I tried to deactivate all plugins to see if some plugin was causing the trouble but there was none.
So, today I change theme completely but problem is still there.

So, I used putty and found that most of CPU was using by mysql and index.php

I not very technical into these things.
I search on Google for index.php server load and found that many people are having these problem.

My CPU usage is fluctuating from 5-150% within seconds (don’t know how this is happening)

I am daily getting 20-25k page-views with 100+ visitors online every time so my site is very important to me.

I really really need to longtime solution to this problem.
Can anyone suggest me what should I do to avoid these?
Any help would be highly appreciated.

Thank You