How do I fix the "No internet access" problem?

[Levitus J]

I have a WiFi network at home and all the PCs are connected via the wireless network. I plugged one of them into the router directly (via a LAN cable) for a few minutes perform an internet speedtest at speedtest.net. However, when I unplugged the laptop from the router to make it wireless again, I got a yellow triangle with an exclamation point on the wifi icon. Hovering the mouse over this, I found that it says its connected to the wifi but states parenthetically: "No internet access".

If i leave the laptop idle for a while and randomly check it back, I have seen the wifi signal return to normal but the very moment I try to search anything in a browser, the little yellow triangle pops up on the wifi icon.

I've tried the following to no avail:
1) Restarting the laptop.
2) Turning the laptop's wifi card off and on.
3) Resetting the router.
4) Shutting down all the other PCs (just in case there was some IP conflict- which there wasn't but i just tried this anyways)
5)Release and renew ip from the command prompt.

I haven't tried setting up a static IP because it just seemed like too far a solution for a problem that never existed before.

Interestingly enough, when I used a LAN connection with the laptop again, while the internet works like normal, i noticed that on another laptop, the wifi icon got the yellow triangle with the exclamation point ie the problem transferred.

I have no idea how to solve this &google hasn't been enlightening.

Please help..!