Why won't my Apple Mac laptop connect to my wifi?

My wifi connects to every other device in the house (Mac family computer, iPhones, iPods etc) but for some reason, despite the password and login information being correct and unaltered ever since I connected to my wifi last year, my laptop is refusing to connect? It's been like this for 3 weeks. I've tried restarting it, renewing the DHCP lease, I updated all necessary software but it still refuses to connect. Occasionally it connects but only for a second or so before logging off again and it's really beginning to annoy me! This has never happened before and it has always functioned fine before now.

I was wondering if anyone could please help me

And yes, as many people have suggested before, I have checked that the wifi is, in fact, on.