You may also want to check your router settings. You may have access control on which doesn't allow new devices to connect. I've forgotten this feature a few times and had many headaches because of it. You may need to whitelist your mac address.
If you know the ip address range you could hard code your ip address (something higher in the range isn't normally assigned automatically). Go into network settings, then connections properties, change from dhcp to manual and type the ip address and subnet (10 beginning ip subnet is 255.0.0.0, 172 beginning ip subnet is 255.255.0.0, 192 beginning ip subnet is 255.255.255.0). Make sure to include the default gateway, if you don't this won't work. If access control is off and this works, try restarting your router, there is a problem with dhcp. It could be the wifi adapter or motherboard are done if this doesn't, further testing would be required.
Edit: i don't know how long it's been since your modem has been up, but powering it off (unplug the modem for 30 seconds... power cycle the router right after this) may help clear up issues. Modems aren't as smart as the rest of the devices for networks, but they can get bogged down to and ignore updates from both the router and isp. Since a reset of your router helped for a time, I'm wondering if this may be modem or router related. If you download netspot from the android store to your phone it will show networks available, channel, signal strength and more. Check signal strength is good where you are attempting to connect, make sure only one or two other networks are using the same channel (default depends on frequency) and if there are more try changing the router channel from automatic to a specific one. Make sure your router has a decent dhcp lease time (about 24hrs, less if you want it to renew more often).