Why would my cellphone not receive/send messages to one particular person, when all other text messages seem to be going through?

I texted a friend this morning and she never got the text – then she texted me back and I never received her message (my phone says no new messages, while her message was definitely sent to me, but apparently not received by me). We’ve called each other on our cellphones, which seem to be working fine for phone calls, and I’ve sent other texts today to people who have gotten them and replied. So why are my texts to one specific person not being delivered?? Is there a reason for this?