No display on putty (connection by serial)

[rainbox]

For using raspberry in my school's wifi, I have to connect it by serial connection.
(I have to get ip because it changes)
Yesterday, I used serial connection in same condition.
However, today, I can not use it.
PuTTY's display is just blank. No message. Just black window.

How can I solve this problem?
I googled this problem. There are just connection tutorials and network connection problems.
No serial connection problems..

I tried to reconnect several times and I failed...

[DirkS]

Are you using the correct COM port?
I found that Windows sometimes changes the COM port number unexpectedly...

[klricks]

If you are using a USB to Serial adapter on your PC then be sure to plug into the same USB port each time. You will likely get a different COM port with each USB port.