Do you happen to know the name of the plant? Also, did you notice a root structure in the pot after the stems had broken off? Looks like the plant could be Cabomba or Limnophila sessiliflora, or potentially hornwort (ceratophyllum demersum). If it's hornwort you could keep it, because it doesn't form a true root structure and will grow either if it's inserted in the substrate or floating. Cambomba is considerably more demanding, and potentially could have died before breaking off (needs more light than 1.5 wpg). Limnophila is a less demanding plant; even if it broke off you might have tried replanting it in the substrate and crossing your fingers.
Was the plant placed directly under the filter outflow? Did it look unhealthy before breaking off? I'm wondering if the plant died first and then broke off, or if it broke off first.