Then you have 2 options. Ask someone else who uses your router for the password -OR- Reset the router and setup a new password and set that new password on all attached devices. Otherwise, we really can't help.
small reset button on the back of the router, hold it for 20 seconds. might need a pen or a paperclip. Router has been reset, you will have to change the name of the network back to whatever you want it, and set up wireless security again.