Internet connection failure issue

hi! I have an I mac that wasnt used for some months, then I used it and unlike any other times I have used this mac, it would not connect to the internet- it connects to the router (the router sees this as i logged into it to check) and all is fine to that point...

but when i go onto safari, it states 'not connected to the server'

now, I have an iphone, and this connects ok to the wireless too, and goes on line no problems. I also had (up until the other day) a work lap top (non mac), which also connected just fine and allowed me onto the web.

I have also logged onto next doors wireless using my Imac, and I can get online no problem!

No settings were changed from the last time the mac was used, and there is no IP conflict (I have checked this with the router)

What is this issue? how can i fix it? there is obviously some setting somewhere specific to my home network that is not right.

Also i my mac wont connect to the internet even if i use the ethernet cable instead of wireless.

I have re-set my router back to factory settings just to see, and still nothing is different!

now that i dont have a work computer anymore, I only have this imac as my sole way to get online (apart form the iphone, which is no good for when i need to do work off this mac! so I really need some help in getting to the bottom of this issue either to solve it myself, or just to know exactly who I need to all out to fix it!!

Can anyone help???