How do I reset my tablet? It says Fastboot mode.

To unlock the bootloader on your Lenovo Tab3 7 Essential (TB3-710I) tablet, you need to follow these steps:
Note: Unlocking the bootloader will erase all data on your device. Make sure to backup your important data before proceeding.

1. Enable Developer Options on your device. To do this, go to Settings > About tablet > tap the Build number 7 times.
2. Enable USB Debugging. To do this, go to Settings > Developer options > enable USB debugging.
3. Download and install the ADB and Fastboot tools on your computer.
4. Connect your tablet to your computer using a USB cable.
5. Open a command prompt window on your computer.
6. In the command prompt window, type the following command and press Enter: adb reboot bootloader, This will reboot your tablet into the bootloader mode.
7. Next, type the following command and press Enter: fastboot oem unlock, This will unlock the bootloader on your tablet.
8. Once the process is complete, reboot your tablet by typing the following command and pressing Enter: fastboot reboot. That's it! Your Lenovo Tab3 7 Essential tablet's bootloader is now unlocked.

You're in Fastboot mode.

Hold Power + Volume Up until the tablet resets.

KEEP HOLDING THE POWER BUTTON UNTIL YOUR PHONES SWITCHES OFF OR PULL OUT THE BATTERY DIRECTLY(If Required) to exit the FAST BOOT MODE and then start your phone normally by pressing only the power button.

Also in case you were trying to enter the recovery mode to wipe cache or hard reset then keep holding the power button and volume DOWN button until you get vibration and release the power button and keep holding the volume DOWN button to enter RECOVERY.

You will be stuck in fastboot if you have failed to flash an img file. You will have to boot a boot.img file using fastboot then download the last update to get your tablet back to normal.
Download the following file onto your computer...
K1_A301_03_03_110829_US.zip
Then unzip on the computer. You will see a file named "boot.img". Make sure your tablet is connected to the computer and you have the fastboot binary. Then run the following command in the directory you just unzipped the file
fastboot boot boot.img
This will allow you to boot up your tablet but the bootloader will STILL be stuck in fastboot. Therefore you will have to update your system. Download the same file above into the Download directory of your tablet. Rename it to update.zip. Dowload the following XML file...
K1_A301_03_03_110829_US.xml
and rename it to update.xml (NOTE: the system file manager won't let you change the extension so just rename to "update". Power off the tablet. Then hold down the Volume + key and hold down power button until it comes on. That will boot the system into recovery and install the update. In a few minutes you will be restored and your tablet will work normally. NOTE: YOU MAY LOSE YOUR DATA and may have to reinstall your apps.
If your boot partition isn't a mess like mine was, you may be able to just flash the system partition from the above zip file by using...
fastboot flash system system.img
fastboot reboot
The key is that a fastboot flash must be successful in order for your device to boot normally.

Source: Lenovo

I would book mark these links. Below.

Copied from :

https://www.kingoapp.com/help/fastboot-mode.htm

What is Fastboot for?

While in fastboot, you can modify the file system images from a computer over a USB connection. Fastboot mode can start on your device even before Android loads, even under the circumstance when Android isn't installed at all. And because of that, fastboot mode is useful for quick-updating the firmware, without having to use a recovery mode. In fact, it's the preferred way to initially install the recovery image. Once you set up, you boot to your phone to fastboot and you can flash image files to the internal memory. Flashing a custom recovery like ClockworkMod is a popular use case. The images you flash don't need to be signed with a particular key, and thus you should use care when flashing. It can also be used for developer operations. Things like erasing partitions and overriding kernel command line options can be done. And all this features make the tool very useful for developing hardware and software solutions, like unlocking the Bootloader of Google' Nexus.
https://www.kingoapp.com/help/fastboot-mode.htm http://forums.androidcentral.com/verizon-droid-dna/252896-how-do-i-turn-off-fast-boot-mode.html

Hello, today I will show you how to fix stuck on android or remove password, for model SC-91JB - SC-74JB
First, connect your tablet to PC.
you need drivers to be able to reset;
download the adb drivers from website;
www.adbdriver.com/downloads/
or click on link below for direct download.
ADB Driver Installer (9.22 MB)
next, install the ADB driver then you will see a USB root on notification area on your PC.
next, download the fastboot click on link below;
https://www.dropbox.com/s/adu0ysny2191kgd/Fastboot.zip?dl=0
install fastboot and you will open a dialog box.
Follow the steps on image below; step one, two and three...

next, command prompt will open and you need to type these commands;
This will reboot and erase password.

If stuck on Android; type commands below (it's the same way like on top image just shorter commands).

> adb devices
> adb shell
> wipe data
> adb reboot
> exit

that is all; your tablet will be completely restored...

thanks and good luck.

Fastboot is associated when you root Android. Here's a site that helps explain it better in detail: The Easiest Way to Install Android ADB and Fastboot Tools on Any OS

Hope this helps you get a better understanding of what it is.

Its under system / developer options settings just untick the fastboot option.

either connect this to a pc/ remove the battery, begin again.

these might be the initial stages of the re-flashing process ie:installing the OS or upgrading when you might connect a usb cable,im not familiar with your brand.