I have had to deal with this issue in the past. These are the solutions that worked for me:
1) Unplug or remove the CMOS battery AND main battery OR check for BIOS jumpers on the motherboard. This will reset the BIOS password (this may not be an option on your model).
2) Contact Dell Support. Give them your Service Tag number. They will need to verify ownership, but will be able to unlock your device.
3) There are several online services that will provide you with a master password for a fee based on your Service Tag number. Do an online search.
4) Take the device into an authorized Dell repair facility. They can unlock the device for you as well.
5) There are lists of master passwords based on your Service Tag and laptop model ( I have had limited success with this option).
6) IF ALL ELSE FAILS - Replace the motherboard. The BIOS is encoded on a chip on the motherboard, which you can replace. For your particular model, it is only about $80. [link|https://totallaptopsolutions.com/collections|website|new_window=true]