My fiance and I, who are in the same room and on the same internet, logged onto comp, I selected Mercy, and he said he wanted to play Hanzo. I told him he needed to choose quickly then. And he was like “I already did?” But my screen was unmoving. And When I tried moving my mouse or doing anything nothing happened and I realized my game was frozen. As soon as his screen moved into “Take Defence” my screen closed and it said I lost connection to the server. I logged back in and hit “rejoin game” as quickly as I could but it was too late. The game shut down, I am banned from Comp for 10 minutes, my SR is way lower, and my endorsement level has dropped.
I’m so incredibly dishearted by this unfair system, where I’m scared to play Comp just because I don’t want to be punished for Blizzard servers dropping me for whatever reason for a second and logging me back in too late. I wasn’t even given 10 seconds to rejoin the game before it closed off and banned me.
There are a multitude of possible causes in your end (local network issues like described in the reply above, and in addition your PC hardware, OS, drivers or other software), so you should try to troubleshoot it first before calling it a Blizzard issue.
Also, don’t be afraid to continue playing (after fixing it or making sure it’s not a recurring problem). When you lose SR due to disconnections, your hidden MMR is not affected which means the match maker will give you matches where you have more than 50% chance of winning, until you’re back at your old SR (assuming your old SR reflected your current skill well enough, i.e. it was close to your hidden MMR). This happened to me too once when I had just managed to grind to a new career high. I got a freak disconnection and lost some SR, guess was I pissed. After that, in the next games I noticed that I got a lot more SR per win than usual and climbed very fast back to where I was (I actually got even higher).
Pluph, since no one explained the details of the rules, I should remind you that the game will cancel at the start of a match when there is a leaver. Rejoining a game without penalty is only possible during certain phases of the game and within a timely manner. If you leave a game either during the pre-game or during the first minute of the game, the game will automatically cancel (remember the 10 second rule does not apply to the leaver, but as a warning to other players that the match will cancel). You can rejoin the match during the initial hero selection screen if you are lucky enough to quickly reconnect though.
Otherwise, you can only rejoin any-time after one-minute of match time has passed up until the match completes. However you only have two minutes to do so without penalty. If you rejoin a match after two-minutes, you will still automatically be marked with a loss an be penalized with a -50 SR cut and a ten-minute suspension. This rule ensures that no one can abusive the leaver system by attempting to return to a match after deliberately leaving in order to avoid such penalties. Furthermore leaving three or more times in a single match will result in an automatic penalty. If you fail to return to a match by the time it ends normally (regardless of match result) you will be penalized.
Unfortunately all disconnections and game crashes counts as leaving the match and will be penalized appropriately. The game server has no way to discern a legitimate disconnection or game crash from one that is forcefully caused by a dishonest player to bypass leaver penalties. You can learn about this policy here:
That being said, if you are disconnecting frequently, avoid competitive play. Remember if you see the message, “Lost Connection to the Server” in the title screen or the message “Failed to Connect to Server” in yellow text in the text chat box, this means there is something that disrupted your connection to the game server and not a problem with the game server itself. There can be a variety of reasons this happens including those that don’t disconnect your computer from the Internet. There is great troubleshooting to start with that can most likely resolve most issues and I encourage you to check them out here:
As always if nothing in that guide works, please reply saying you completed these steps and additional advice can be provided. Good luck!
In the event that there’s actually a server crash during an actual match, you and all other players will be taken back to the main menu and see the message “Server closed due to an unexpected error.” in the text message box, and in most cases a match will restart on a new map.
In the event that there is a problem with one of Blizzard Network providers, such as being affected by a DDoS attack, there will be notifications posted on the Battle App launcher and made through the @BlizzardCS Twitter feed.
When you lose SR due to disconnections, your hidden MMR is not affected which means the match maker will give you matches where you have more than 50% chance of winning, until you’re back at your old SR (assuming your old SR reflected your current skill well enough, i.e. it was close to your hidden MMR). This happened to me too once when I had just managed to grind to a new career high. I got a freak disconnection and lost some SR, guess was I pissed. 
After that, in the next games I noticed that I got a lot more SR per win than usual and climbed very fast back to where I was (I actually got even higher).