gdgt

Try changing your encryption type in your wireless security settings. On a D-Link router that I used to live with, changing it from WEP to WPA/WPA2 somehow allowed different devices to connect. Also, update your firmware. If that doesn't work, install dd-wrt (looks like it's compatible).

www.dd­-wrt.com­/site­/support­/router­-database

Make sure to install the right version of dd-wrt that fits your routers revision number or you'll risk bricking it.

The firmware on lower end D-Link routers are notoriously bad. If that doesn't work, it may be time to consider a router upgrade.

There could be many reasons for this. First, are you on a desktop or a laptop or a portable device like a tablet or a phone? Are there any other computers or devices connected to the internet at the same time when you experience this problem?

This is usually NOT due to software malfunctions/malware/viruses

The two most common reason for such issues are:
1. You have a laptop and when it comes out of standby or hibernate the wifi device is either slow to respond or completely unresponsive.
In this case you would not be able to connect to wifi on the laptop.
Solution: Shut down your laptop between uses.

2. Your ISP changes your IP address and your router fails to renew the DHCP address.
In this case you would see a connection to wifi but no internet connection is available.
Solution: Since every router has different configurations and releasing the DHCP can be a bit of a chore for most users, I recommended that you unplug your router for 30 seconds, then plug it back in. This will renew the DHCP and you should be able to get online.

Other possibilities include:
Faulty wifi card in a device or notebook
Faulty router
ISP rebooting the router frequently
Interference from microwaves or portable phones or other wifi hotspots in the area broadcasting on the same channel
Wifi antenna misalignment
Wifi antenna too close to something metal
Sunspots (yes they can create problems)