On another device open a browser and go to https://account.microsoft.com/devices/recoverykey, then log in with the same Microsoft account you used when you set up the Surface.
The page will list all BitLocker recovery keys backed up to that account. Compare the first 8 digits of the Recovery Key ID shown on the blue BitLocker screen of your Surface with the Key IDs listed on the web page. When one matches, use the corresponding 48-digit recovery key to unlock the device.
If it's not there You may have set up the Surface with a different Microsoft account (an old email, for example). Try signing in with any other accounts you may have had.
If the key truly cannot be found, you can reset the Surface using Microsoft's official recovery image. You can find the Surface recovery images at: https://support.microsoft.com/surface-recovery-image