This is usually a sign that the Macbook can't detect your hard drive. So, something happened while you were working that caused your hard drive to either stop spinning, or just to not be recognized by your system anymore.
Unless you have experience taking apart Macbooks, and have the appropriate drive sled to offload data from your (possibly) failing drive, I'd highly recommend taking it into a Mac service place so that they can copy the data off your drive before it completely fails (if it hasn't already). That's your best bet for data recovery. While they've got it apart, they can put in a new drive (I'd recommend an SSD upgrade if you have a spinning disk currently, since it'll speed your machine WAY up).
The other option is if you have another Mac and a thunderbolt (or firewire if this is an old mac) cable, you can try repeatedly rebooting while holding down the T-key. This boots into Firewire/Thunderbolt Target Disk Mode, which basically turns your laptop into an expensive external hard drive. When hard drives start to fail, occasionally you'll be able to reboot several times and they'll work again. Do that, and if/when it works, then plug into the other Mac and immediately copy off all of your most important files, followed by your less important files, since the drive could fail again at any time.
Hope this helps and you can recover your data!