Wifi not working on iphone 3g

hi well last night i was all connected up to my router and on the net. everything was working correctly and there was no problems. then suddenly the connection drops on my phone which i thought was strange because the router is only on my desk. so i turned the wifi on my phone off and then back on and since then i get the error cannot connect to network *****. on all the other devices i can connect fine e.g. my macbook imac and ipod touch.

thank you for your time