SMS Not Sending to One Number

I have a Pixel 3 and my husband has a pixel 1. We have been texting successfully since I got this phone last year. Today, mid-conversation my phone stopped sending SMS text messages to him. I can still send SMS to other contacts and can send my husband MMS texts. Any thoughts on how to fix? I'm using the standard pixel messaging app and have already restarted my phone and cleared the cache in the app. Thanks in advance! 

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