I can't connect to the internet using wifi—only by ethernet

For starters, I'd suggest you verify whether your wireless network is still working. Are your other wireless devices (tablet, phone, etc.) still able to connect? If not, you'll want to troubleshoot your router/access point—power cycle it, check the settings/configuration, etc. The procedure for this is pretty basic but will vary a bit depending on which router you have. Remember that you can't connect unless both the router and your laptop are using the same network name (SSID) and password.

If your other wireless devices can connect but your laptop can't, try removing the network in Windows and then re-adding it as if you were connecting for the first time. The procedure goes something like this:

1. Open up the Control Panel
2. Start Network and Sharing Center
3. In the Tasks pane, click Manage wireless networks
4. Right-click the connection you want to delete, and then click Remove network

Once the network is removed, re-add it as if for the first time (right-click the wireless icon in the lower right corner of the screen, select your network from the list, and type in your network password).

If that doesn't work, try downloading and reinstalling the wireless network driver for your system here. Reboot and try connecting again.

If you still have trouble, you may need to revert to an earlier restore point in Windows.

Report back and let us know if any of that works! If not, it might be time to physically reseat the wireless hardware and/or replace it.