4s not sending sms to one contact but sends i message. other contacts are ok

ive got a 4s. phone was fine sending and recieving txt sms last night but i sent a sms this morning and the "send " was blue and popup cannot send message enable to send.

no setings were touched. and my contact has changed anything.

I can send a sms fine to any other contact i have.

checked all phone settings and makes no differance.

if I switch on i message then mine get sent as an i mesage but the reply i get says text message above there message.

my contact hasnt blocked me and is on a iphone

any ideas. thanks.