Unfortunately the rules do require that if you leave a game and fail to return by the time the match ends (or within 2 minutes, whichever is sooner), that you will be marked with the loss and penalty. You can learn about this policy here:
I am presuming this was a simple one-time connection issue, and if so you should not have to worry again. However if you are disconnecting frequently, please remember to avoid Competitive Play and work to troubleshoot your connection. Assistance is always available in the technical support forum if needed.