Why can't I use wifi on my android phone (Moto G Lolipop) even though it works perfect on my laptop?

So I've been facing this problem lately, earlier the wifi worked perfect on both my laptop and my phone but suddenly it has stopped working on my phone (an exclamation mark is displayed near the wifi signal on my phone).

Sometimes when I use the 'forget wifi option' on the phone and retype the password, it is able to connect whatsapp to the internet but nothing else. (the exclamation mark is still displayed near the wifi signal though. ) I face a similar problem with another android phone (Samsung S III neo)

This problem has emerged lately even though I've not changed any settings or so of the wifi. I've tried switching on/off both the modem and the phone but with no luck. It works perfectly fine with the laptop but not on the phone (even though I'm a foot away from the modem)

Please help. Thanks.