There are several things you can do if you can't get connected to your WiFi hotpot or router.
1). The TF cannot use channels above 11. But some channels work better than others. 1 and 6 are popular and work well for many people. If you can get connected at all, download
WiFi Analyzer This app will give you an idea what channels may be available or over lapping with a nearby router.
2). Restart your router. This sounds simple but has been the solution for many people. Routers frequently accumulate garbage in the system that can be dumped by removing the power cord for at least 30 seconds then plugging it back in. After the router has made a connection, restart your TF
3). As is the case with some other devices, the TF doesn't do a great job of telling you there is an error when inputting your password or passphrase. If the TF says it is trying to connect for more than 10-15 seconds you may have mistyped the required information. Long press the connection and tell it to forget your credentials and try again.
4). I have found that the TF does not work well or at all with WEP encryption so your best bet is to use WPA/WPA2 or equivalent. This is better security than WEP anyway. If you still can't connect but see a strong signal try no encryption long enough to determine that it is possible to connect and adjust accordingly.
5). In order to narrow the problem down, if all else is failing, try connecting to a public hotspot or a friend's router.
6) If 1-5 don't help, do a cold start of the TF. With the TF turned off, hold the volume button in the down position and while holding the volume button hold the power button. Hold both until you clearly see some white text in the upper left corner of the screen then let go of both buttons. You should then see 2 icons on the screen with the one on the right flashing. Wait until the TF restarts. Note, double check all your settings as this causes some to get lost, like screen timeouts for instance.
These instructions work for the Transformer and the Transformer Prime but also should work for the Slider