This is a fairly common thing to happen if your computer remembers your network connection. Some element of your hotspot changed, and your computer is trying to connect using the old configuration. Here's what you need to do:
1) Open up your connection manager and view the available networks.
2) Right click on your internet connection to your phone.
3) Choose one of the following options (varies by computer type): Forget, delete, uncheck connect automatically, etc. Essentially
what you are doing is removing the record that this connection ever existed.
4) Set it up and reconnect it again. You can also try changing the security type (if steps 1-3 don't work) in the 3G mobile hotspot settings.
Let us know if this works for you.