Hi, I am Dave, I will help you with this.
Are you dual booting Windows with Linux?
If you are, this is caused by a reported problem in the August update if you can get into Windows, either uninstall the August update, or open Command Prompt as administrator and run this command, then press Enter.
reg add HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\SecureBoot\SBAT /v OptOut /d 1 /t REG_DWORD
Then close Command Prompt