im sorry to hear that.
Zebra danios were my first fish (after doing a fishless cycle) and within 2 weeks i lost one to mouth fungus, found him stuck in the plants. next was a baby replacement i got. he got in a fight with another danios and lost some scales from it. died.
The third one was real tragic. she got trapped between the filter and the glass after the suckers failed and it slid, taking that perfect moment in time to pin her. she suffered extreme damage and later died in my hospital bucket about 3 days later.
they are very hardy fish, but if one takes a dint in its immune system, it will go. watch your others for any odd symptoms and check your water stats daily for the next week to watch for any fluctuations.