IMessage Problem

i messaging my friend off my ipod . butt all of a sudden my imessages stop working and everything i click on his name it says number not registed with imessages . when i know he is .i cant message anyone on imessages anymore and i dont know why . can someone please help me figure this out .i have tried testing my network and i have tried resting my whole ipod touch i have tried reseting my imessages