How do I fix it on yahoo messenger when it comes up at a blank box?

Okay i was talking to someone for a bit and got other ims that showed a blank box, so I tried to sign off and restart the computer so I could see the im but after getting back in touch kept getting this blank box for an im...now iming people this is still appearing. Is this just yahoo or is it my computer? if it is my computer how can I fix it?